Merge branch 'i18n' into develop

This merge adds the internationalization features, the final required
feature for 7Units version 1.0.0.

11 files changed, 760 insertions, 357 deletions

diff --git a/.classpath b/.classpath
index d4e759d..02a1f84 100644
--- a/.classpath
+++ b/.classpath
@@ -25,6 +25,6 @@
 			<attribute name="module" value="true"/>
 		</attributes>
 	</classpathentry>
-	<classpathentry kind="con" path="org.eclipse.buildship.core.gradleclasspathcontainer"/>
+	<classpathentry kind="con" path="org.eclipse.jdt.junit.JUNIT_CONTAINER/5"/>
 	<classpathentry kind="output" path="bin/default"/>
 </classpath>
diff --git a/src/main/java/sevenUnitsGUI/Presenter.java b/src/main/java/sevenUnitsGUI/Presenter.java
index c46ee53..3a039a7 100644
--- a/src/main/java/sevenUnitsGUI/Presenter.java
+++ b/src/main/java/sevenUnitsGUI/Presenter.java
@@ -16,12 +16,12 @@
  */
 package sevenUnitsGUI;
 
-import java.io.BufferedWriter;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -57,7 +57,7 @@ import sevenUnitsGUI.StandardDisplayRules.UncertaintyBased;
 
 /**
  * An object that handles interactions between the view and the backend code
- * 
+ *
  * @author Adrien Hopkins
  * @since 2021-12-15
  */
@@ -77,10 +77,23 @@ public final class Presenter {
 	/** A Predicate that returns true iff the argument is a full base unit */
 	private static final Predicate<Unit> IS_FULL_BASE = unit -> unit instanceof LinearUnit
 			&& ((LinearUnit) unit).isBase();
-	
+	/**
+	 * The default locale, used in two situations:
+	 * <ul>
+	 * <li>If no text is available in your locale, uses text from this locale.
+	 * <li>Users are initialized with this locale.
+	 * </ul>
+	 */
+	static final String DEFAULT_LOCALE = "en";
+
+	private static final List<String> LOCAL_LOCALES = List.of("en", "fr");
+	private static final Path USER_LOCALES_DIR = userConfigDir()
+			.resolve("SevenUnits").resolve("locales");
+
+
 	/**
 	 * Adds default units and dimensions to a database.
-	 * 
+	 *
 	 * @param database database to add to
 	 * @since 2019-04-14
 	 * @since v0.2.0
@@ -99,20 +112,20 @@ public final class Presenter {
 		// nonlinear units - must be loaded manually
 		database.addUnit("tempCelsius", Metric.CELSIUS);
 		database.addUnit("tempFahrenheit", BritishImperial.FAHRENHEIT);
-		
+
 		// load initial dimensions
 		database.addDimension("Length", Metric.Dimensions.LENGTH);
 		database.addDimension("Mass", Metric.Dimensions.MASS);
 		database.addDimension("Time", Metric.Dimensions.TIME);
 		database.addDimension("Temperature", Metric.Dimensions.TEMPERATURE);
 	}
-	
+
 	private static String displayRuleToString(
 			Function<UncertainDouble, String> numberDisplayRule) {
 		if (numberDisplayRule instanceof FixedDecimals)
 			return String.format("FIXED_DECIMALS %d",
 					((FixedDecimals) numberDisplayRule).decimalPlaces());
-		else if (numberDisplayRule instanceof FixedPrecision)
+		if (numberDisplayRule instanceof FixedPrecision)
 			return String.format("FIXED_PRECISION %d",
 					((FixedPrecision) numberDisplayRule).significantFigures());
 		else if (numberDisplayRule instanceof UncertaintyBased)
@@ -120,21 +133,21 @@ public final class Presenter {
 		else
 			return numberDisplayRule.toString();
 	}
-	
+
 	/**
 	 * Determines where to wrap {@code toWrap} with a max line length of
 	 * {@code maxLineLength}. If no good spot is found, returns -1.
 	 *
 	 * @since 2024-08-22
 	 */
-	private static final int findLineSplit(String toWrap, int maxLineLength) {
-		for (int i = maxLineLength - 1; i >= 0; i--) {
+	private static int findLineSplit(String toWrap, int maxLineLength) {
+		for (var i = maxLineLength - 1; i >= 0; i--) {
 			if (Character.isWhitespace(toWrap.charAt(i)))
 				return i;
 		}
 		return -1;
 	}
-	
+
 	/**
 	 * Gets the text of a resource file as a set of strings (each one is one line
 	 * of the text).
@@ -143,11 +156,11 @@ public final class Presenter {
 	 * @return contents of file
 	 * @since 2021-03-27
 	 */
-	private static final List<String> getLinesFromResource(String filename) {
+	private static List<String> getLinesFromResource(String filename) {
 		final List<String> lines = new ArrayList<>();
-		
-		try (InputStream stream = inputStream(filename);
-				Scanner scanner = new Scanner(stream)) {
+
+		try (var stream = inputStream(filename);
+				var scanner = new Scanner(stream)) {
 			while (scanner.hasNextLine()) {
 				lines.add(scanner.nextLine());
 			}
@@ -155,10 +168,10 @@ public final class Presenter {
 			throw new AssertionError(
 					"Error occurred while loading file " + filename, e);
 		}
-		
+
 		return lines;
 	}
-	
+
 	/**
 	 * Gets an input stream for a resource file.
 	 *
@@ -166,98 +179,97 @@ public final class Presenter {
 	 * @return obtained Path
 	 * @since 2021-03-27
 	 */
-	private static final InputStream inputStream(String filepath) {
+	private static InputStream inputStream(String filepath) {
 		return Presenter.class.getResourceAsStream(filepath);
 	}
-	
+
 	/**
 	 * Convert a linear unit value to a string, where the number is rounded to
 	 * the nearest integer.
 	 *
 	 * @since 2024-08-16
 	 */
-	private static final String linearUnitValueIntToString(LinearUnitValue uv) {
+	private static String linearUnitValueIntToString(LinearUnitValue uv) {
 		return Long.toString(Math.round(uv.getValueExact())) + " " + uv.getUnit();
 	}
-	
+
 	private static Map.Entry<String, String> parseSettingLine(String line) {
-		final int equalsIndex = line.indexOf('=');
+		final var equalsIndex = line.indexOf('=');
 		if (equalsIndex == -1)
 			throw new IllegalStateException(
 					"Settings file is malformed at line: " + line);
-		
-		final String param = line.substring(0, equalsIndex);
-		final String value = line.substring(equalsIndex + 1);
-		
+
+		final var param = line.substring(0, equalsIndex);
+		final var value = line.substring(equalsIndex + 1);
+
 		return Map.entry(param, value);
 	}
-	
+
 	/** Gets a Path from a pathname in the config file. */
 	private static Path pathFromConfig(String pathname) {
 		return CONFIG_FILE.getParent().resolve(pathname);
 	}
-	
+
 	// ====== SETTINGS ======
-	
+
 	private static String searchRuleToString(
 			Function<Map.Entry<String, LinearUnit>, Map<String, LinearUnit>> searchRule) {
 		if (PrefixSearchRule.NO_PREFIXES.equals(searchRule))
 			return "NO_PREFIXES";
-		else if (PrefixSearchRule.COMMON_PREFIXES.equals(searchRule))
+		if (PrefixSearchRule.COMMON_PREFIXES.equals(searchRule))
 			return "COMMON_PREFIXES";
 		else if (PrefixSearchRule.ALL_METRIC_PREFIXES.equals(searchRule))
 			return "ALL_METRIC_PREFIXES";
 		else
 			return searchRule.toString();
 	}
-	
+
 	/**
 	 * @return true iff a and b have any elements in common
 	 * @since 2022-04-19
 	 */
-	private static final boolean sharesAnyElements(Set<?> a, Set<?> b) {
+	private static boolean sharesAnyElements(Set<?> a, Set<?> b) {
 		for (final Object e : a) {
 			if (b.contains(e))
 				return true;
 		}
 		return false;
 	}
-	
-	private static final Path userConfigDir() {
+
+	private static Path userConfigDir() {
 		if (System.getProperty("os.name").startsWith("Windows")) {
-			final String envFolder = System.getenv("LOCALAPPDATA");
+			final var envFolder = System.getenv("LOCALAPPDATA");
 			if (envFolder == null || "".equals(envFolder))
 				return Path.of(System.getenv("USERPROFILE"), "AppData", "Local");
 			else
 				return Path.of(envFolder);
-		} else {
-			final String envFolder = System.getenv("XDG_CONFIG_HOME");
-			if (envFolder == null || "".equals(envFolder))
-				return Path.of(System.getenv("HOME"), ".config");
-			else
-				return Path.of(envFolder);
 		}
+		final var envFolder = System.getenv("XDG_CONFIG_HOME");
+		if (envFolder == null || "".equals(envFolder))
+			return Path.of(System.getenv("HOME"), ".config");
+		else
+			return Path.of(envFolder);
 	}
-	
+
 	/**
 	 * @return {@code line} with any comments removed.
 	 * @since 2021-03-13
 	 */
-	private static final String withoutComments(String line) {
-		final int index = line.indexOf('#');
+	private static String withoutComments(String line) {
+		final var index = line.indexOf('#');
 		return index == -1 ? line : line.substring(0, index);
 	}
-	
+
 	/**
 	 * Wraps a string, ensuring no line is longer than {@code maxLineLength}.
-	 * 
+	 *
 	 * @since 2024-08-22
 	 */
-	private static final String wrapString(String toWrap, int maxLineLength) {
-		final StringBuilder wrapped = new StringBuilder(toWrap.length());
-		String remaining = toWrap;
+	private static String wrapString(String toWrap, int maxLineLength) {
+		final var wrapped = new StringBuilder(toWrap.length());
+		var remaining = toWrap;
 		while (remaining.length() > maxLineLength) {
-			final int spot = findLineSplit(toWrap, maxLineLength);
+			final var spot = findLineSplit(toWrap, maxLineLength);
 			if (spot == -1) {
 				wrapped.append(remaining.substring(0, maxLineLength));
 				wrapped.append("-\n");
@@ -271,23 +283,23 @@ public final class Presenter {
 		wrapped.append(remaining);
 		return wrapped.toString();
 	}
-	
+
 	/**
 	 * The view that this presenter communicates with
 	 */
 	private final View view;
-	
+
 	/**
 	 * The database that this presenter communicates with (effectively the model)
 	 */
 	final UnitDatabase database;
-	
+
 	/**
 	 * The rule used for parsing input numbers. Any number-string inputted into
 	 * this program will be parsed using this method. <b>Not implemented yet.</b>
 	 */
 	private Function<String, UncertainDouble> numberParsingRule;
-	
+
 	/**
 	 * The rule used for displaying the results of unit conversions. The result
 	 * of unit conversions will be put into this function, and the resulting
@@ -295,35 +307,41 @@ public final class Presenter {
 	 */
 	private Function<UncertainDouble, String> numberDisplayRule = StandardDisplayRules
 			.uncertaintyBased();
-	
+
 	/**
 	 * A predicate that determines whether or not a certain combination of
 	 * prefixes is allowed. If it returns false, a combination of prefixes will
 	 * not be allowed. Prefixes are put in the list from right to left.
 	 */
 	private Predicate<List<UnitPrefix>> prefixRepetitionRule = DefaultPrefixRepetitionRule.NO_RESTRICTION;
-	
+
 	/**
 	 * A rule that accepts a prefixless name-unit pair and returns a map mapping
 	 * names to prefixed versions of that unit (including the unit itself) that
 	 * should be searchable.
 	 */
 	private Function<Map.Entry<String, LinearUnit>, Map<String, LinearUnit>> searchRule = PrefixSearchRule.NO_PREFIXES;
-	
+
 	/**
 	 * The set of units that is considered neither metric nor nonmetric for the
 	 * purposes of the metric-imperial one-way conversion. These units are
 	 * included in both From and To, even if One Way Conversion is enabled.
 	 */
 	private final Set<String> metricExceptions;
-	
+
+	/** maps locale names (e.g. 'en') to key-text maps */
+	final Map<String, Map<String, String>> locales;
+
+	/** name of locale in locales to use */
+	String userLocale;
+
 	/**
 	 * If this is true, views that show units as a list will have metric units
 	 * removed from the From unit list and imperial/USC units removed from the To
 	 * unit list.
 	 */
 	private boolean oneWayConversionEnabled = false;
-	
+
 	/**
 	 * If this is false, duplicate units and prefixes will be removed from the
 	 * unit view in views that show units as a list to choose from.
@@ -331,39 +349,88 @@ public final class Presenter {
 	private boolean showDuplicates = false;
 	
 	/**
+	 * The default unit, prefix, dimension and exception data will only be loaded
+	 * if this variable is true.
+	 */
+	private boolean useDefaultDatafiles = true;
+	
+	/** Custom unit datafiles that will be loaded by {@link #reloadData} */
+	private final Set<Path> customUnitFiles = new HashSet<>();
+	/** Custom dimension datafiles that will be loaded by {@link #reloadData} */
+	private final Set<Path> customDimensionFiles = new HashSet<>();
+	/** Custom exception datafiles that will be loaded by {@link #reloadData} */
+	private final Set<Path> customExceptionFiles = new HashSet<>();
+
+	/**
 	 * Creates a Presenter
-	 * 
+	 *
 	 * @param view the view that this presenter communicates with
 	 * @since 2021-12-15
 	 */
 	public Presenter(View view) {
 		this.view = view;
 		this.database = new UnitDatabase();
+		this.metricExceptions = new HashSet<>();
+
+		this.locales = this.loadLocales();
+		this.userLocale = DEFAULT_LOCALE;
+
+		// set default settings temporarily
+		if (Files.exists(CONFIG_FILE)) {
+			this.loadSettings(CONFIG_FILE);
+		}
+
+		this.reloadData();
+
+		// print out unit counts
+		System.out.println(this.loadStatMsg());
+	}
+	
+	/**
+	 * Clears then reloads all unit, prefix, dimension and exception data.
+	 */
+	public void reloadData() {
+		this.database.clear();
+		this.metricExceptions.clear();
 		addDefaults(this.database);
 		
+		if (this.useDefaultDatafiles) {
+			this.loadDefaultData();
+		}
+		
+		this.customUnitFiles.forEach(
+				path -> this.handleLoadErrors(this.database.loadUnitsFile(path)));
+		this.customDimensionFiles.forEach(
+				path -> this.handleLoadErrors(this.database.loadDimensionFile(path)));
+		this.customExceptionFiles.forEach(this::loadExceptionFile);
+	}
+	
+	/**
+	 * Load units, prefixes and dimensions from the default files.
+	 */
+	private void loadDefaultData() {
 		// load units and prefixes
-		try (final InputStream units = inputStream(DEFAULT_UNITS_FILEPATH)) {
+		try (final var units = inputStream(DEFAULT_UNITS_FILEPATH)) {
 			this.handleLoadErrors(this.database.loadUnitsFromStream(units));
 		} catch (final IOException e) {
 			throw new AssertionError("Loading of unitsfile.txt failed.", e);
 		}
-		
+
 		// load dimensions
-		try (final InputStream dimensions = inputStream(
+		try (final var dimensions = inputStream(
 				DEFAULT_DIMENSIONS_FILEPATH)) {
 			this.handleLoadErrors(
 					this.database.loadDimensionsFromStream(dimensions));
 		} catch (final IOException e) {
 			throw new AssertionError("Loading of dimensionfile.txt failed.", e);
 		}
-		
+
 		// load metric exceptions
 		try {
-			this.metricExceptions = new HashSet<>();
-			try (InputStream exceptions = inputStream(DEFAULT_EXCEPTIONS_FILEPATH);
-					Scanner scanner = new Scanner(exceptions)) {
+			try (var exceptions = inputStream(DEFAULT_EXCEPTIONS_FILEPATH);
+					var scanner = new Scanner(exceptions)) {
 				while (scanner.hasNextLine()) {
-					final String line = Presenter
+					final var line = Presenter
 							.withoutComments(scanner.nextLine());
 					if (!line.isBlank()) {
 						this.metricExceptions.add(line);
@@ -374,16 +441,8 @@ public final class Presenter {
 			throw new AssertionError("Loading of metric_exceptions.txt failed.",
 					e);
 		}
-		
-		// set default settings temporarily
-		if (Files.exists(CONFIG_FILE)) {
-			this.loadSettings(CONFIG_FILE);
-		}
-		
-		// print out unit counts
-		System.out.println(this.loadStatMsg());
 	}
-	
+
 	/**
 	 * Applies a search rule to an entry in a name-unit map.
 	 *
@@ -391,23 +450,23 @@ public final class Presenter {
 	 * @return stream of entries, ready for flat-mapping
 	 * @since 2022-07-06
 	 */
-	private final Stream<Map.Entry<String, Unit>> applySearchRule(
+	private Stream<Map.Entry<String, Unit>> applySearchRule(
 			Map.Entry<String, Unit> e) {
-		final Unit u = e.getValue();
+		final var u = e.getValue();
 		if (u instanceof LinearUnit) {
-			final String name = e.getKey();
+			final var name = e.getKey();
 			final Map.Entry<String, LinearUnit> linearEntry = Map.entry(name,
 					(LinearUnit) u);
 			return this.searchRule.apply(linearEntry).entrySet().stream().map(
 					entry -> Map.entry(entry.getKey(), (Unit) entry.getValue()));
-		} else
-			return Stream.of(e);
+		}
+		return Stream.of(e);
 	}
-	
+
 	/**
 	 * Converts from the view's input expression to its output expression.
 	 * Displays an error message if any of the required fields are invalid.
-	 * 
+	 *
 	 * @throws UnsupportedOperationException if the view does not support
 	 *                                       expression-based conversion (does
 	 *                                       not implement
@@ -415,49 +474,46 @@ public final class Presenter {
 	 * @since 2021-12-15
 	 */
 	public void convertExpressions() {
-		if (this.view instanceof ExpressionConversionView) {
-			final ExpressionConversionView xcview = (ExpressionConversionView) this.view;
-			
-			final String fromExpression = xcview.getFromExpression();
-			final String toExpression = xcview.getToExpression();
-			
-			// expressions must not be empty
-			if (fromExpression.isEmpty()) {
-				this.view.showErrorMessage("Parse Error",
-						"Please enter a unit expression in the From: box.");
-				return;
-			}
-			if (toExpression.isEmpty()) {
-				this.view.showErrorMessage("Parse Error",
-						"Please enter a unit expression in the To: box.");
-				return;
-			}
-			
-			final Optional<UnitConversionRecord> uc;
-			if (this.database.containsUnitSetName(toExpression)) {
-				uc = this.convertExpressionToNamedMultiUnit(fromExpression,
-						toExpression);
-			} else if (toExpression.contains(";")) {
-				final String[] toExpressions = toExpression.split(";");
-				uc = this.convertExpressionToMultiUnit(fromExpression,
-						toExpressions);
-			} else {
-				uc = this.convertExpressionToExpression(fromExpression,
-						toExpression);
-			}
-			
-			uc.ifPresent(xcview::showExpressionConversionOutput);
-		} else
+		if (!(this.view instanceof ExpressionConversionView))
 			throw new UnsupportedOperationException(
 					"This function can only be called when the view is an ExpressionConversionView");
+		final var xcview = (ExpressionConversionView) this.view;
+
+		final var fromExpression = xcview.getFromExpression();
+		final var toExpression = xcview.getToExpression();
+
+		// expressions must not be empty
+		if (fromExpression.isEmpty()) {
+			this.view.showErrorMessage("Parse Error",
+					"Please enter a unit expression in the From: box.");
+			return;
+		}
+		if (toExpression.isEmpty()) {
+			this.view.showErrorMessage("Parse Error",
+					"Please enter a unit expression in the To: box.");
+			return;
+		}
+
+		final Optional<UnitConversionRecord> uc;
+		if (this.database.containsUnitSetName(toExpression)) {
+			uc = this.convertExpressionToNamedMultiUnit(fromExpression,
+					toExpression);
+		} else if (toExpression.contains(";")) {
+			final var toExpressions = toExpression.split(";");
+			uc = this.convertExpressionToMultiUnit(fromExpression, toExpressions);
+		} else {
+			uc = this.convertExpressionToExpression(fromExpression, toExpression);
+		}
+
+		uc.ifPresent(xcview::showExpressionConversionOutput);
 	}
-	
+
 	/**
 	 * Converts a unit expression to another expression.
-	 * 
+	 *
 	 * If an error happened, it is shown to the view and Optional.empty() is
 	 * returned.
-	 * 
+	 *
 	 * @since 2024-08-15
 	 */
 	private Optional<UnitConversionRecord> convertExpressionToExpression(
@@ -479,34 +535,33 @@ public final class Presenter {
 					"Could not recognize text in To entry: " + e.getMessage());
 			return Optional.empty();
 		}
-		
+
 		// convert and show output
-		if (from.getUnit().canConvertTo(to)) {
-			final UncertainDouble uncertainValue;
-			
-			// uncertainty is meaningless for non-linear units, so we will have
-			// to erase uncertainty information for them
-			if (to instanceof LinearUnit) {
-				final var toLinear = (LinearUnit) to;
-				uncertainValue = from.convertTo(toLinear).getValue();
-			} else {
-				final double value = from.asUnitValue().convertTo(to).getValue();
-				uncertainValue = UncertainDouble.of(value, 0);
-			}
-			
-			final UnitConversionRecord uc = UnitConversionRecord.valueOf(
-					fromExpression, toExpression, "",
-					this.numberDisplayRule.apply(uncertainValue));
-			return Optional.of(uc);
-		} else {
+		if (!from.getUnit().canConvertTo(to)) {
 			this.view.showErrorMessage("Conversion Error",
 					"Cannot convert between \"" + fromExpression + "\" and \""
 							+ toExpression + "\".");
 			return Optional.empty();
 		}
-		
+		final UncertainDouble uncertainValue;
+
+		// uncertainty is meaningless for non-linear units, so we will have
+		// to erase uncertainty information for them
+		if (to instanceof LinearUnit) {
+			final var toLinear = (LinearUnit) to;
+			uncertainValue = from.convertTo(toLinear).getValue();
+		} else {
+			final var value = from.asUnitValue().convertTo(to).getValue();
+			uncertainValue = UncertainDouble.of(value, 0);
+		}
+
+		final var uc = UnitConversionRecord.valueOf(
+				fromExpression, toExpression, "",
+				this.numberDisplayRule.apply(uncertainValue));
+		return Optional.of(uc);
+
 	}
-	
+
 	/**
 	 * Convert an expression to a MultiUnit. If an error happened, it is shown to
 	 * the view and Optional.empty() is returned.
@@ -523,27 +578,26 @@ public final class Presenter {
 					"Could not recognize text in From entry: " + e.getMessage());
 			return Optional.empty();
 		}
-		
+
 		final List<LinearUnit> toUnits = new ArrayList<>(toExpressions.length);
-		for (int i = 0; i < toExpressions.length; i++) {
+		for (final String toExpression : toExpressions) {
 			try {
-				final Unit toI = this.database
-						.getUnitFromExpression(toExpressions[i].trim());
-				if (toI instanceof LinearUnit) {
-					toUnits.add((LinearUnit) toI);
-				} else {
+				final var toI = this.database
+						.getUnitFromExpression(toExpression.trim());
+				if (!(toI instanceof LinearUnit)) {
 					this.view.showErrorMessage("Unit Type Error",
 							"Units separated by ';' must be linear; " + toI
 									+ " is not.");
 					return Optional.empty();
 				}
+				toUnits.add((LinearUnit) toI);
 			} catch (final IllegalArgumentException | NoSuchElementException e) {
 				this.view.showErrorMessage("Parse Error",
 						"Could not recognize text in To entry: " + e.getMessage());
 				return Optional.empty();
 			}
 		}
-		
+
 		final List<LinearUnitValue> toValues;
 		try {
 			toValues = from.convertToMultiple(toUnits);
@@ -552,12 +606,12 @@ public final class Presenter {
 					"Invalid units separated by ';': " + e.getMessage());
 			return Optional.empty();
 		}
-		
-		final String toExpression = this.linearUnitValueSumToString(toValues);
+
+		final var toExpression = this.linearUnitValueSumToString(toValues);
 		return Optional.of(
 				UnitConversionRecord.valueOf(fromExpression, toExpression, "", ""));
 	}
-	
+
 	/**
 	 * Convert an expression to a MultiUnit with a name from the database. If an
 	 * error happened, it is shown to the view and Optional.empty() is returned.
@@ -574,8 +628,8 @@ public final class Presenter {
 					"Could not recognize text in From entry: " + e.getMessage());
 			return Optional.empty();
 		}
-		
-		final List<LinearUnit> toUnits = this.database.getUnitSet(toName);
+
+		final var toUnits = this.database.getUnitSet(toName);
 		final List<LinearUnitValue> toValues;
 		try {
 			toValues = from.convertToMultiple(toUnits);
@@ -584,16 +638,16 @@ public final class Presenter {
 					"Invalid units separated by ';': " + e.getMessage());
 			return Optional.empty();
 		}
-		
-		final String toExpression = this.linearUnitValueSumToString(toValues);
+
+		final var toExpression = this.linearUnitValueSumToString(toValues);
 		return Optional.of(
 				UnitConversionRecord.valueOf(fromExpression, toExpression, "", ""));
 	}
-	
+
 	/**
 	 * Converts from the view's input unit to its output unit. Displays an error
 	 * message if any of the required fields are invalid.
-	 * 
+	 *
 	 * @throws UnsupportedOperationException if the view does not support
 	 *                                       unit-based conversion (does not
 	 *                                       implement
@@ -601,64 +655,61 @@ public final class Presenter {
 	 * @since 2021-12-15
 	 */
 	public void convertUnits() {
-		if (this.view instanceof UnitConversionView) {
-			final UnitConversionView ucview = (UnitConversionView) this.view;
-			
-			final Optional<String> fromUnitOptional = ucview.getFromSelection();
-			final Optional<String> toUnitOptional = ucview.getToSelection();
-			final String inputValueString = ucview.getInputValue();
-			
-			// extract values from optionals
-			final String fromUnitString, toUnitString;
-			if (fromUnitOptional.isPresent()) {
-				fromUnitString = fromUnitOptional.orElseThrow();
-			} else {
-				this.view.showErrorMessage("Unit Selection Error",
-						"Please specify a From unit");
-				return;
-			}
-			if (toUnitOptional.isPresent()) {
-				toUnitString = toUnitOptional.orElseThrow();
-			} else {
-				this.view.showErrorMessage("Unit Selection Error",
-						"Please specify a To unit");
-				return;
-			}
-			
-			// convert strings to data, checking if anything is invalid
-			final Unit fromUnit;
-			final UncertainDouble uncertainValue;
-			
-			if (this.database.containsUnitName(fromUnitString)) {
-				fromUnit = this.database.getUnit(fromUnitString);
-			} else
-				throw this.viewError("Nonexistent From unit: %s", fromUnitString);
-			try {
-				uncertainValue = UncertainDouble
-						.fromRoundedString(inputValueString);
-			} catch (final NumberFormatException e) {
-				this.view.showErrorMessage("Value Error",
-						"Invalid value " + inputValueString);
-				return;
-			}
-			if (this.database.containsUnitName(toUnitString)) {
-				final Unit toUnit = this.database.getUnit(toUnitString);
-				ucview.showUnitConversionOutput(
-						this.convertUnitToUnit(fromUnitString, toUnitString,
-								inputValueString, fromUnit, toUnit, uncertainValue));
-			} else if (this.database.containsUnitSetName(toUnitString)) {
-				final List<LinearUnit> toMulti = this.database
-						.getUnitSet(toUnitString);
-				ucview.showUnitConversionOutput(
-						this.convertUnitToMulti(fromUnitString, inputValueString,
-								fromUnit, toMulti, uncertainValue));
-			} else
-				throw this.viewError("Nonexistent To unit: %s", toUnitString);
-		} else
+		if (!(this.view instanceof UnitConversionView))
 			throw new UnsupportedOperationException(
 					"This function can only be called when the view is a UnitConversionView.");
+		final var ucview = (UnitConversionView) this.view;
+
+		final var fromUnitOptional = ucview.getFromSelection();
+		final var toUnitOptional = ucview.getToSelection();
+		final var inputValueString = ucview.getInputValue();
+
+		// extract values from optionals
+		final String fromUnitString, toUnitString;
+		if (fromUnitOptional.isPresent()) {
+			fromUnitString = fromUnitOptional.orElseThrow();
+		} else {
+			this.view.showErrorMessage("Unit Selection Error",
+					"Please specify a From unit");
+			return;
+		}
+		if (toUnitOptional.isPresent()) {
+			toUnitString = toUnitOptional.orElseThrow();
+		} else {
+			this.view.showErrorMessage("Unit Selection Error",
+					"Please specify a To unit");
+			return;
+		}
+
+		// convert strings to data, checking if anything is invalid
+		final Unit fromUnit;
+		final UncertainDouble uncertainValue;
+
+		if (this.database.containsUnitName(fromUnitString)) {
+			fromUnit = this.database.getUnit(fromUnitString);
+		} else
+			throw this.viewError("Nonexistent From unit: %s", fromUnitString);
+		try {
+			uncertainValue = UncertainDouble.fromRoundedString(inputValueString);
+		} catch (final NumberFormatException e) {
+			this.view.showErrorMessage("Value Error",
+					"Invalid value " + inputValueString);
+			return;
+		}
+		if (this.database.containsUnitName(toUnitString)) {
+			final var toUnit = this.database.getUnit(toUnitString);
+			ucview.showUnitConversionOutput(
+					this.convertUnitToUnit(fromUnitString, toUnitString,
+							inputValueString, fromUnit, toUnit, uncertainValue));
+		} else if (this.database.containsUnitSetName(toUnitString)) {
+			final var toMulti = this.database
+					.getUnitSet(toUnitString);
+			ucview.showUnitConversionOutput(this.convertUnitToMulti(fromUnitString,
+					inputValueString, fromUnit, toMulti, uncertainValue));
+		} else
+			throw this.viewError("Nonexistent To unit: %s", toUnitString);
 	}
-	
+
 	private UnitConversionRecord convertUnitToMulti(String fromUnitString,
 			String inputValueString, Unit fromUnit, List<LinearUnit> toMulti,
 			UncertainDouble uncertainValue) {
@@ -667,7 +718,7 @@ public final class Presenter {
 				throw this.viewError("Could not convert between %s and %s",
 						fromUnit, toUnit);
 		}
-		
+
 		final LinearUnitValue initValue;
 		if (fromUnit instanceof LinearUnit) {
 			final var fromLinear = (LinearUnit) fromUnit;
@@ -676,46 +727,46 @@ public final class Presenter {
 			initValue = UnitValue.of(fromUnit, uncertainValue.value())
 					.convertToBase(NameSymbol.EMPTY);
 		}
-		
-		final List<LinearUnitValue> converted = initValue
+
+		final var converted = initValue
 				.convertToMultiple(toMulti);
-		final String toExpression = this.linearUnitValueSumToString(converted);
+		final var toExpression = this.linearUnitValueSumToString(converted);
 		return UnitConversionRecord.valueOf(fromUnitString, toExpression,
 				inputValueString, "");
-		
+
 	}
-	
+
 	private UnitConversionRecord convertUnitToUnit(String fromUnitString,
 			String toUnitString, String inputValueString, Unit fromUnit,
 			Unit toUnit, UncertainDouble uncertainValue) {
 		if (!fromUnit.canConvertTo(toUnit))
 			throw this.viewError("Could not convert between %s and %s", fromUnit,
 					toUnit);
-			
+
 		// convert - we will need to erase uncertainty for non-linear units, so
 		// we need to treat linear and non-linear units differently
 		final String outputValueString;
 		if (fromUnit instanceof LinearUnit && toUnit instanceof LinearUnit) {
-			final LinearUnit fromLinear = (LinearUnit) fromUnit;
-			final LinearUnit toLinear = (LinearUnit) toUnit;
-			final LinearUnitValue initialValue = LinearUnitValue.of(fromLinear,
+			final var fromLinear = (LinearUnit) fromUnit;
+			final var toLinear = (LinearUnit) toUnit;
+			final var initialValue = LinearUnitValue.of(fromLinear,
 					uncertainValue);
-			final LinearUnitValue converted = initialValue.convertTo(toLinear);
-			
+			final var converted = initialValue.convertTo(toLinear);
+
 			outputValueString = this.numberDisplayRule.apply(converted.getValue());
 		} else {
-			final UnitValue initialValue = UnitValue.of(fromUnit,
+			final var initialValue = UnitValue.of(fromUnit,
 					uncertainValue.value());
-			final UnitValue converted = initialValue.convertTo(toUnit);
-			
+			final var converted = initialValue.convertTo(toUnit);
+
 			outputValueString = this.numberDisplayRule
 					.apply(UncertainDouble.of(converted.getValue(), 0));
 		}
-		
+
 		return UnitConversionRecord.valueOf(fromUnitString, toUnitString,
 				inputValueString, outputValueString);
 	}
-	
+
 	/**
 	 * @return true iff duplicate units are shown in unit lists
 	 * @since 2022-03-30
@@ -723,18 +774,46 @@ public final class Presenter {
 	public boolean duplicatesShown() {
 		return this.showDuplicates;
 	}
-	
+
 	/**
 	 * @return text in About file
 	 * @since 2022-02-19
 	 */
-	public final String getAboutText() {
-		return Presenter.getLinesFromResource("/about.txt").stream()
+	public String getAboutText() {
+		final Path customFilepath = Presenter.pathFromConfig(
+				"about/" + this.userLocale + ".txt");
+		if (Files.exists(customFilepath)) {
+			try {
+				return formatAboutText(Files.lines(customFilepath));
+			} catch (IOException e) {
+				final String filename = String.format("/about/%s.txt", this.userLocale);
+				return formatAboutText(Presenter.getLinesFromResource(filename).stream());
+			}
+		} else if (LOCAL_LOCALES.contains(this.userLocale)) {
+			final String filename = String.format("/about/%s.txt", this.userLocale);
+			return formatAboutText(Presenter.getLinesFromResource(filename).stream());
+		} else {
+			final String filename = String.format("/about/%s.txt", DEFAULT_LOCALE);
+			return formatAboutText(Presenter.getLinesFromResource(filename).stream());
+		}
+		
+	}
+	
+	private String formatAboutText(Stream<String> rawLines) {
+		return rawLines
 				.map(Presenter::withoutComments).collect(Collectors.joining("\n"))
 				.replaceAll("\\[VERSION\\]", ProgramInfo.VERSION.toString())
 				.replaceAll("\\[LOADSTATS\\]", wrapString(this.loadStatMsg(), 72));
 	}
-	
+
+	/**
+	 * @return set of all locales available to select
+	 * @since 2025-02-21
+	 */
+	public Set<String> getAvailableLocales() {
+		return this.locales.keySet();
+	}
+
 	/**
 	 * Gets a name for this dimension using the database
 	 *
@@ -742,14 +821,28 @@ public final class Presenter {
 	 * @return name of dimension
 	 * @since 2022-04-16
 	 */
-	final String getDimensionName(ObjectProduct<BaseDimension> dimension) {
+	String getDimensionName(ObjectProduct<BaseDimension> dimension) {
 		// find this dimension in the database and get its name
 		// if it isn't there, use the dimension's toString instead
 		return this.database.dimensionMap().values().stream()
 				.filter(d -> d.equals(dimension)).findAny().map(Nameable::getName)
 				.orElse(dimension.toString(Nameable::getName));
 	}
-	
+
+	/**
+	 * Gets the correct text for a provided ID. If this text is available in the
+	 * user's locale, that text is provided. Otherwise, text is taken from the
+	 * system default locale {@link #DEFAULT_LOCALE}.
+	 *
+	 * @param textID ID of text to get (used in locale files)
+	 * @return text to be displayed
+	 */
+	public String getLocalizedText(String textID) {
+		final var userLocale = this.locales.get(this.userLocale);
+		final var defaultLocale = this.locales.get(DEFAULT_LOCALE);
+		return userLocale.getOrDefault(textID, defaultLocale.get(textID));
+	}
+
 	/**
 	 * @return the rule that is used by this presenter to convert numbers into
 	 *         strings
@@ -758,7 +851,7 @@ public final class Presenter {
 	public Function<UncertainDouble, String> getNumberDisplayRule() {
 		return this.numberDisplayRule;
 	}
-	
+
 	/**
 	 * @return the rule that is used by this presenter to convert strings into
 	 *         numbers
@@ -768,7 +861,7 @@ public final class Presenter {
 	private Function<String, UncertainDouble> getNumberParsingRule() {
 		return this.numberParsingRule;
 	}
-	
+
 	/**
 	 * @return the rule that determines whether a set of prefixes is valid
 	 * @since 2022-04-19
@@ -776,7 +869,7 @@ public final class Presenter {
 	public Predicate<List<UnitPrefix>> getPrefixRepetitionRule() {
 		return this.prefixRepetitionRule;
 	}
-	
+
 	/**
 	 * @return the rule that determines which units are prefixed
 	 * @since 2022-07-08
@@ -784,7 +877,7 @@ public final class Presenter {
 	public Function<Map.Entry<String, LinearUnit>, Map<String, LinearUnit>> getSearchRule() {
 		return this.searchRule;
 	}
-	
+
 	/**
 	 * @return a search rule that shows all single prefixes
 	 * @since 2022-07-08
@@ -793,7 +886,15 @@ public final class Presenter {
 		return PrefixSearchRule.getCoherentOnlyRule(
 				new HashSet<>(this.database.prefixMap(true).values()));
 	}
-	
+
+	/**
+	 * @return user's selected locale
+	 * @since 2025-02-21
+	 */
+	public String getUserLocale() {
+		return userLocale;
+	}
+
 	/**
 	 * @return the view associated with this presenter
 	 * @since 2022-04-19
@@ -801,7 +902,7 @@ public final class Presenter {
 	public View getView() {
 		return this.view;
 	}
-	
+
 	/**
 	 * Accepts a list of errors. If that list is non-empty, prints an error
 	 * message and alerts the user.
@@ -810,7 +911,7 @@ public final class Presenter {
 	 */
 	private void handleLoadErrors(List<LoadingException> errors) {
 		if (!errors.isEmpty()) {
-			final String errorMessage = String.format(
+			final var errorMessage = String.format(
 					"%d error(s) happened while loading file:\n%s\n", errors.size(),
 					errors.stream().map(Throwable::getMessage)
 							.collect(Collectors.joining("\n")));
@@ -819,13 +920,13 @@ public final class Presenter {
 					errorMessage);
 		}
 	}
-	
+
 	/**
 	 * @return whether or not the provided unit is semi-metric (i.e. an
 	 *         exception)
 	 * @since 2022-04-16
 	 */
-	final boolean isSemiMetric(Unit u) {
+	boolean isSemiMetric(Unit u) {
 		// determine if u is an exception
 		final var primaryName = u.getPrimaryName();
 		final var symbol = u.getSymbol();
@@ -835,7 +936,7 @@ public final class Presenter {
 						&& this.metricExceptions.contains(symbol.orElseThrow())
 				|| sharesAnyElements(this.metricExceptions, u.getOtherNames());
 	}
-	
+
 	/**
 	 * Convert a list of LinearUnitValues that you would get from a unit-set
 	 * conversion to a string. All but the last have their numbers rendered as
@@ -844,18 +945,17 @@ public final class Presenter {
 	 *
 	 * @since 2024-08-16
 	 */
-	private final String linearUnitValueSumToString(
-			List<LinearUnitValue> values) {
-		final String integerPart = values.subList(0, values.size() - 1).stream()
+	private String linearUnitValueSumToString(List<LinearUnitValue> values) {
+		final var integerPart = values.subList(0, values.size() - 1).stream()
 				.map(Presenter::linearUnitValueIntToString)
 				.collect(Collectors.joining(" + "));
-		final LinearUnitValue last = values.get(values.size() - 1);
+		final var last = values.get(values.size() - 1);
 		return integerPart + " + " + this.numberDisplayRule.apply(last.getValue())
 				+ " " + last.getUnit();
 	}
-	
+
 	private void loadExceptionFile(Path exceptionFile) {
-		try (Stream<String> lines = Files.lines(exceptionFile)) {
+		try (var lines = Files.lines(exceptionFile)) {
 			lines.map(Presenter::withoutComments)
 					.forEach(this.metricExceptions::add);
 		} catch (final IOException e) {
@@ -864,7 +964,54 @@ public final class Presenter {
 							+ exceptionFile + "\": " + e.getLocalizedMessage());
 		}
 	}
+
+	/**
+	 * Loads all available locales, including custom ones, into a map.
+	 * @return map containing locales
+	 */
+	private Map<String, Map<String, String>> loadLocales() {
+		final Map<String, Map<String, String>> locales = new HashMap<>();
+		for (final String localeName : LOCAL_LOCALES) {
+			final Map<String, String> locale = new HashMap<>();
+			String filename = "/locales/" + localeName + ".txt";
+			getLinesFromResource(filename).forEach(line -> addLocaleLine(locale, line));
+			locales.put(localeName, locale);	
+		}
+		
+		if (Files.exists(USER_LOCALES_DIR)) {
+			try {
+				Files.list(USER_LOCALES_DIR).forEach(
+						localeFile -> {
+							try {
+								addLocaleFile(locales, localeFile);
+							} catch (IOException e) {
+								e.printStackTrace();
+							}
+						});
+			} catch (IOException e) {
+				e.printStackTrace();
+			}
+		}
+		return locales;
+	}
 	
+	private void addLocaleFile(Map<String, Map<String, String>> locales, Path file) throws IOException {
+		final Map<String, String> locale = new HashMap<>();
+		String fileName = file.getName(file.getNameCount()-1).toString();
+		String localeName = fileName.substring(0, fileName.length()-4);
+		Files.lines(file).forEach(line -> addLocaleLine(locale, line));
+		locales.put(localeName, locale);
+	}
+	
+	private void addLocaleLine(Map<String, String> locale, String line) {
+		String[] parts = line.split("=", 2);
+		if (parts.length < 2) {
+			return;
+		}
+			
+		locale.put(parts[0], parts[1]);
+	}
+
 	/**
 	 * Loads settings from the user's settings file and applies them to the
 	 * presenter.
@@ -873,23 +1020,25 @@ public final class Presenter {
 	 * @since 2021-12-15
 	 */
 	void loadSettings(Path settingsFile) {
+		this.customDimensionFiles.clear();
+		this.customExceptionFiles.clear();
+		this.customUnitFiles.clear();
+
 		for (final Map.Entry<String, String> setting : this
 				.settingsFromFile(settingsFile)) {
-			final String value = setting.getValue();
-			
+			final var value = setting.getValue();
+
 			switch (setting.getKey()) {
 			// set manually to avoid the unnecessary saving of the non-manual
 			// methods
 			case "custom_dimension_file":
-				this.handleLoadErrors(
-						this.database.loadDimensionFile(pathFromConfig(value)));
+				this.customDimensionFiles.add(pathFromConfig(value));
 				break;
 			case "custom_exception_file":
-				this.loadExceptionFile(pathFromConfig(value));
+				this.customExceptionFiles.add(pathFromConfig(value));
 				break;
 			case "custom_unit_file":
-				this.handleLoadErrors(
-						this.database.loadUnitsFile(pathFromConfig(value)));
+				this.customUnitFiles.add(pathFromConfig(value));
 				break;
 			case "number_display_rule":
 				this.setDisplayRuleFromString(value);
@@ -900,73 +1049,78 @@ public final class Presenter {
 				this.database.setPrefixRepetitionRule(this.prefixRepetitionRule);
 				break;
 			case "one_way":
-				this.oneWayConversionEnabled = Boolean.valueOf(value);
+				this.oneWayConversionEnabled = Boolean.parseBoolean(value);
 				break;
 			case "include_duplicates":
-				this.showDuplicates = Boolean.valueOf(value);
+				this.showDuplicates = Boolean.parseBoolean(value);
 				break;
 			case "search_prefix_rule":
 				this.setSearchRuleFromString(value);
 				break;
+			case "locale":
+				this.userLocale = value;
+				break;
 			default:
 				System.err.printf("Warning: unrecognized setting \"%s\".%n",
 						setting.getKey());
 				break;
 			}
 		}
-		
+
 		if (this.view.getPresenter() != null) {
 			this.updateView();
 		}
 	}
-	
+
 	/**
 	 * @return a message showing how much stuff has been loaded
 	 * @since 2024-08-22
 	 */
 	private String loadStatMsg() {
-		return String.format(
-				"Successfully loaded %d unique units with %d names (%d base units), %d unique prefixes with %d names, %d unit sets, and %d named dimensions.",
-				this.database.unitMapPrefixless(false).size(),
-				this.database.unitMapPrefixless(true).size(),
-				this.database.unitMapPrefixless(false).values().stream()
-						.filter(IS_FULL_BASE).count(),
-				this.database.prefixMap(false).size(),
-				this.database.prefixMap(true).size(),
-				this.database.unitSetMap().size(),
-				this.database.dimensionMap().size());
+		return this.getLocalizedText("load_stat_msg")
+				.replace("[u]", Integer.toString(
+						this.database.unitMapPrefixless(false).size()))
+				.replace("[un]", Integer.toString(
+						this.database.unitMapPrefixless(true).size()))
+				.replace("[b]", Long.toString(this.database.unitMapPrefixless(false)
+						.values().stream().filter(IS_FULL_BASE).count()))
+				.replace("[p]", Integer.toString(this.database.prefixMap(false).size()))
+				.replace("[pn]", Integer.toString(this.database.prefixMap(true).size()))
+				.replace("[s]", Integer.toString(this.database.unitSetMap().size()))
+				.replace("[d]", Integer.toString(this.database.dimensionMap().size()));
 	}
-	
+
 	/**
 	 * @return true iff the One-Way Conversion feature is available (views that
 	 *         show units as a list will have metric units removed from the From
 	 *         unit list and imperial/USC units removed from the To unit list)
-	 * 
+	 *
 	 * @since 2022-03-30
 	 */
 	public boolean oneWayConversionEnabled() {
 		return this.oneWayConversionEnabled;
 	}
-	
+
 	/**
 	 * Completes creation of the presenter. This part of the initialization
 	 * depends on the view's functions, so it cannot be run if the components
 	 * they depend on are not created yet.
-	 * 
+	 *
 	 * @since 2022-02-26
 	 */
 	public void postViewInitialize() {
 		// unit conversion specific stuff
 		if (this.view instanceof UnitConversionView) {
-			final UnitConversionView ucview = (UnitConversionView) this.view;
+			final var ucview = (UnitConversionView) this.view;
 			ucview.setDimensionNames(this.database.dimensionMap().keySet());
 		}
-		
+
 		this.updateView();
+		this.view.updateText();
 	}
-	
+
 	void prefixSelected() {
-		final Optional<String> selectedPrefixName = this.view
+		final var selectedPrefixName = this.view
 				.getViewedPrefixName();
 		final Optional<UnitPrefix> selectedPrefix = selectedPrefixName
 				.map(name -> this.database.containsPrefixName(name)
@@ -976,16 +1130,16 @@ public final class Presenter {
 				.ifPresent(prefix -> this.view.showPrefix(prefix.getNameSymbol(),
 						String.valueOf(prefix.getMultiplier())));
 	}
-	
+
 	/**
 	 * Saves the presenter's current settings to the config file, creating it if
 	 * it doesn't exist.
-	 * 
+	 *
 	 * @return false iff the presenter could not write to the file
 	 * @since 2022-04-19
 	 */
 	public boolean saveSettings() {
-		final Path configDir = CONFIG_FILE.getParent();
+		final var configDir = CONFIG_FILE.getParent();
 		if (!Files.exists(configDir)) {
 			try {
 				Files.createDirectories(configDir);
@@ -993,19 +1147,19 @@ public final class Presenter {
 				return false;
 			}
 		}
-		
+
 		return this.writeSettings(CONFIG_FILE);
 	}
-	
+
 	private void setDisplayRuleFromString(String ruleString) {
-		final String[] tokens = ruleString.split(" ");
+		final var tokens = ruleString.split(" ");
 		switch (tokens[0]) {
 		case "FIXED_DECIMALS":
-			final int decimals = Integer.parseInt(tokens[1]);
+			final var decimals = Integer.parseInt(tokens[1]);
 			this.numberDisplayRule = StandardDisplayRules.fixedDecimals(decimals);
 			break;
 		case "FIXED_PRECISION":
-			final int sigDigs = Integer.parseInt(tokens[1]);
+			final var sigDigs = Integer.parseInt(tokens[1]);
 			this.numberDisplayRule = StandardDisplayRules.fixedPrecision(sigDigs);
 			break;
 		case "UNCERTAINTY_BASED":
@@ -1017,7 +1171,7 @@ public final class Presenter {
 			break;
 		}
 	}
-	
+
 	/**
 	 * @param numberDisplayRule the new rule that will be used by this presenter
 	 *                          to convert numbers into strings
@@ -1027,7 +1181,7 @@ public final class Presenter {
 			Function<UncertainDouble, String> numberDisplayRule) {
 		this.numberDisplayRule = numberDisplayRule;
 	}
-	
+
 	/**
 	 * @param numberParsingRule the new rule that will be used by this presenter
 	 *                          to convert strings into numbers
@@ -1038,7 +1192,7 @@ public final class Presenter {
 			Function<String, UncertainDouble> numberParsingRule) {
 		this.numberParsingRule = numberParsingRule;
 	}
-	
+
 	/**
 	 * @param oneWayConversionEnabled whether not one-way conversion should be
 	 *                                enabled
@@ -1049,7 +1203,7 @@ public final class Presenter {
 		this.oneWayConversionEnabled = oneWayConversionEnabled;
 		this.updateView();
 	}
-	
+
 	/**
 	 * @param prefixRepetitionRule the rule that determines whether a set of
 	 *                             prefixes is valid
@@ -1060,7 +1214,7 @@ public final class Presenter {
 		this.prefixRepetitionRule = prefixRepetitionRule;
 		this.database.setPrefixRepetitionRule(prefixRepetitionRule);
 	}
-	
+
 	/**
 	 * @param searchRule A rule that accepts a prefixless name-unit pair and
 	 *                   returns a map mapping names to prefixed versions of that
@@ -1072,7 +1226,7 @@ public final class Presenter {
 			Function<Map.Entry<String, LinearUnit>, Map<String, LinearUnit>> searchRule) {
 		this.searchRule = searchRule;
 	}
-	
+
 	private void setSearchRuleFromString(String ruleString) {
 		switch (ruleString) {
 		case "NO_PREFIXES":
@@ -1090,7 +1244,7 @@ public final class Presenter {
 					ruleString);
 		}
 	}
-	
+
 	/**
 	 * @param showDuplicateUnits whether or not duplicate units should be shown
 	 * @since 2022-03-30
@@ -1099,9 +1253,9 @@ public final class Presenter {
 		this.showDuplicates = showDuplicateUnits;
 		this.updateView();
 	}
-	
+
 	private List<Map.Entry<String, String>> settingsFromFile(Path settingsFile) {
-		try (Stream<String> lines = Files.lines(settingsFile)) {
+		try (var lines = Files.lines(settingsFile)) {
 			return lines.map(Presenter::withoutComments)
 					.filter(line -> !line.isBlank()).map(Presenter::parseSettingLine)
 					.toList();
@@ -1113,6 +1267,26 @@ public final class Presenter {
 	}
 	
 	/**
+	 * Sets whether or not the default datafiles will be loaded.
+	 * This method automatically updates the view's units.
+	 */
+	public void setUseDefaultDatafiles(boolean useDefaultDatafiles) {
+		this.useDefaultDatafiles = useDefaultDatafiles;
+		this.reloadData();
+		this.updateView();
+	}
+
+	/**
+	 * Sets the user's locale, updating the view.
+	 *
+	 * @param userLocale locale to use
+	 */
+	public void setUserLocale(String userLocale) {
+		this.userLocale = userLocale;
+		this.view.updateText();
+	}
+
+	/**
 	 * Shows a unit in the unit viewer
 	 *
 	 * @param u unit to show
@@ -1120,53 +1294,53 @@ public final class Presenter {
 	 */
 	private void showUnit(Unit u) {
 		final var nameSymbol = u.getNameSymbol();
-		final boolean isBase = u instanceof BaseUnit
+		final var isBase = u instanceof BaseUnit
 				|| u instanceof LinearUnit && ((LinearUnit) u).isBase();
 		final var definition = isBase ? "(Base unit)" : u.toDefinitionString();
 		final var dimensionString = this.getDimensionName(u.getDimension());
 		final var unitType = UnitType.getType(u, this::isSemiMetric);
 		this.view.showUnit(nameSymbol, definition, dimensionString, unitType);
 	}
-	
+
 	/**
 	 * Runs whenever a unit name is selected in the unit viewer. Gets the
 	 * description of a unit and displays it.
-	 * 
+	 *
 	 * @since 2022-04-10
 	 */
 	void unitNameSelected() {
 		// get selected unit, if it's there and valid
-		final Optional<String> selectedUnitName = this.view.getViewedUnitName();
+		final var selectedUnitName = this.view.getViewedUnitName();
 		final Optional<Unit> selectedUnit = selectedUnitName
 				.map(unitName -> this.database.containsUnitName(unitName)
 						? this.database.getUnit(unitName)
 						: null);
 		selectedUnit.ifPresent(this::showUnit);
 	}
-	
+
 	/**
 	 * Updates the view's From and To units, if it has some
-	 * 
+	 *
 	 * @since 2021-12-15
 	 */
 	public void updateView() {
 		if (this.view instanceof UnitConversionView) {
-			final UnitConversionView ucview = (UnitConversionView) this.view;
+			final var ucview = (UnitConversionView) this.view;
 			final var selectedDimensionName = ucview.getSelectedDimensionName();
-			
+
 			// load units & prefixes into viewers
 			this.view.setViewableUnitNames(
 					this.database.unitMapPrefixless(this.showDuplicates).keySet());
 			this.view.setViewablePrefixNames(
 					this.database.prefixMap(this.showDuplicates).keySet());
-			
+
 			// get From and To units
 			var fromUnits = this.database.unitMapPrefixless(this.showDuplicates)
 					.entrySet().stream();
 			var toUnits = this.database.unitMapPrefixless(this.showDuplicates)
 					.entrySet().stream();
 			var unitSets = this.database.unitSetMap().entrySet().stream();
-			
+
 			// filter by dimension, if one is selected
 			if (selectedDimensionName.isPresent()) {
 				final var viewDimension = this.database
@@ -1178,7 +1352,7 @@ public final class Presenter {
 				unitSets = unitSets.filter(us -> viewDimension
 						.equals(us.getValue().get(0).getDimension()));
 			}
-			
+
 			// filter by unit type, if desired
 			if (this.oneWayConversionEnabled) {
 				fromUnits = fromUnits.filter(u -> UnitType.getType(u.getValue(),
@@ -1189,7 +1363,7 @@ public final class Presenter {
 				unitSets = unitSets
 						.filter(us -> this.metricExceptions.contains(us.getKey()));
 			}
-			
+
 			// set unit names
 			ucview.setFromUnitNames(fromUnits.flatMap(this::applySearchRule)
 					.map(Map.Entry::getKey).collect(Collectors.toSet()));
@@ -1203,6 +1377,13 @@ public final class Presenter {
 	}
 	
 	/**
+	 * @return true iff the default datafiles are being used
+	 */
+	public boolean usingDefaultDatafiles() {
+		return this.useDefaultDatafiles;
+	}
+
+	/**
 	 * @param message message to add
 	 * @param args    string formatting arguments for message
 	 * @return AssertionError stating that an error has happened in the view's
@@ -1213,15 +1394,15 @@ public final class Presenter {
 		return new AssertionError("View Programming Error (from " + this.view
 				+ "): " + String.format(message, args));
 	}
-	
+
 	/**
 	 * Saves the presenter's settings to the user settings file.
-	 * 
+	 *
 	 * @param settingsFile file settings should be saved to
 	 * @since 2021-12-15
 	 */
 	boolean writeSettings(Path settingsFile) {
-		try (BufferedWriter writer = Files.newBufferedWriter(settingsFile)) {
+		try (var writer = Files.newBufferedWriter(settingsFile)) {
 			writer.write(String.format("number_display_rule=%s\n",
 					displayRuleToString(this.numberDisplayRule)));
 			writer.write(
@@ -1232,6 +1413,7 @@ public final class Presenter {
 					String.format("include_duplicates=%s\n", this.showDuplicates));
 			writer.write(String.format("search_prefix_rule=%s\n",
 					searchRuleToString(this.searchRule)));
+			writer.write(String.format("locale=%s\n", this.userLocale));
 			return true;
 		} catch (final IOException e) {
 			e.printStackTrace();
diff --git a/src/main/java/sevenUnitsGUI/TabbedView.java b/src/main/java/sevenUnitsGUI/TabbedView.java
index 6542541..9850aac 100644
--- a/src/main/java/sevenUnitsGUI/TabbedView.java
+++ b/src/main/java/sevenUnitsGUI/TabbedView.java
@@ -24,13 +24,16 @@ import java.awt.event.ItemEvent;
 import java.awt.event.KeyEvent;
 import java.util.AbstractSet;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.OptionalInt;
 import java.util.Set;
+import java.util.function.Consumer;
 import java.util.function.Function;
 
 import javax.swing.BorderFactory;
@@ -195,10 +198,14 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 	/** The text box for prefix data in the prefix viewer */
 	private final JTextArea prefixTextBox;
 	
-	// SETTINGS STUFF
+	// INFO & SETTINGS STUFF
+	final JTextArea infoTextArea;
+	private final JComboBox<String> localeSelector;
 	private StandardRoundingType roundingType;
 	private int precision;
 	
+	private final Map<String, Consumer<String>> localizedTextSetters;
+	
 	/**
 	 * Creates the view and makes it visible to the user
 	 * 
@@ -218,16 +225,20 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		
 		// initialize important components
 		this.presenter = new Presenter(this);
-		this.frame = new JFrame("7Units " + ProgramInfo.VERSION);
+		this.frame = new JFrame("7Units (Unlocalized)");
 		this.frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
 		
 		// master components (those that contain everything else within them)
 		this.masterPane = new JTabbedPane();
 		this.frame.add(this.masterPane);
 		
+		this.localizedTextSetters = new HashMap<>();
+		
 		// ============ UNIT CONVERSION TAB ============
 		final JPanel convertUnitPanel = new JPanel();
 		this.masterPane.addTab("Convert Units", convertUnitPanel);
+		this.localizedTextSetters.put("tv.convert_units.title",
+				txt -> this.masterPane.setTitleAt(0, txt));
 		this.masterPane.setMnemonicAt(0, KeyEvent.VK_U);
 		convertUnitPanel.setLayout(new BorderLayout());
 		
@@ -263,7 +274,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			outputPanel.setLayout(new BorderLayout());
 			outputPanel.setBorder(new EmptyBorder(3, 6, 6, 6));
 			
-			final JLabel valuePrompt = new JLabel("Value to convert: ");
+			final JLabel valuePrompt = new JLabel();
+			this.localizedTextSetters.put("tv.convert_units.value_prompt", 
+					valuePrompt::setText);
 			outputPanel.add(valuePrompt, BorderLayout.LINE_START);
 			
 			this.valueInput = new JTextField();
@@ -271,6 +284,8 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			
 			// conversion button
 			this.convertUnitButton = new JButton("Convert");
+			this.localizedTextSetters.put("tv.convert_units.convert_btn", 
+					this.convertUnitButton::setText);
 			outputPanel.add(this.convertUnitButton, BorderLayout.LINE_END);
 			this.convertUnitButton
 					.addActionListener(e -> this.presenter.convertUnits());
@@ -286,20 +301,26 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		final JPanel convertExpressionPanel = new JPanel();
 		this.masterPane.addTab("Convert Unit Expressions",
 				convertExpressionPanel);
+		this.localizedTextSetters.put("tv.convert_expressions.title", 
+				txt -> this.masterPane.setTitleAt(1, txt));
 		this.masterPane.setMnemonicAt(1, KeyEvent.VK_E);
 		convertExpressionPanel.setLayout(new GridLayout(4, 1));
 		
 		// from and to expressions
 		this.fromEntry = new JTextField();
 		convertExpressionPanel.add(this.fromEntry);
-		this.fromEntry.setBorder(BorderFactory.createTitledBorder("From"));
+		this.localizedTextSetters.put("tv.convert_expressions.from", 
+				txt -> this.fromEntry.setBorder(BorderFactory.createTitledBorder(txt)));
 		
 		this.toEntry = new JTextField();
 		convertExpressionPanel.add(this.toEntry);
-		this.toEntry.setBorder(BorderFactory.createTitledBorder("To"));
+		this.localizedTextSetters.put("tv.convert_expressions.to", 
+				txt -> this.toEntry.setBorder(BorderFactory.createTitledBorder(txt)));
 		
 		// button to convert
-		this.convertExpressionButton = new JButton("Convert");
+		this.convertExpressionButton = new JButton();
+		this.localizedTextSetters.put("tv.convert_expressions.convert_btn",
+				this.convertExpressionButton::setText);
 		convertExpressionPanel.add(this.convertExpressionButton);
 		
 		this.convertExpressionButton
@@ -309,13 +330,15 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		// output of conversion
 		this.expressionOutput = new JTextArea(2, 32);
 		convertExpressionPanel.add(this.expressionOutput);
-		this.expressionOutput
-				.setBorder(BorderFactory.createTitledBorder("Output"));
+		this.localizedTextSetters.put("tv.convert_expressions.output", 
+				txt -> this.expressionOutput.setBorder(BorderFactory.createTitledBorder(txt)));
 		this.expressionOutput.setEditable(false);
 		
 		// =========== UNIT VIEWER ===========
 		final JPanel unitLookupPanel = new JPanel();
 		this.masterPane.addTab("Unit Viewer", unitLookupPanel);
+		this.localizedTextSetters.put("tv.unit_viewer.title", 
+				txt -> this.masterPane.setTitleAt(2, txt));
 		this.masterPane.setMnemonicAt(2, KeyEvent.VK_V);
 		unitLookupPanel.setLayout(new GridLayout());
 		
@@ -333,6 +356,8 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		// ============ PREFIX VIEWER =============
 		final JPanel prefixLookupPanel = new JPanel();
 		this.masterPane.addTab("Prefix Viewer", prefixLookupPanel);
+		this.localizedTextSetters.put("tv.prefix_viewer.title", 
+				txt -> this.masterPane.setTitleAt(3, txt));
 		this.masterPane.setMnemonicAt(3, KeyEvent.VK_P);
 		prefixLookupPanel.setLayout(new GridLayout(1, 2));
 		
@@ -353,19 +378,20 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		this.masterPane.addTab("\uD83D\uDEC8", // info (i) character
 				new JScrollPane(infoPanel));
 		
-		final JTextArea infoTextArea = new JTextArea();
-		infoTextArea.setEditable(false);
-		infoTextArea.setOpaque(false);
-		infoPanel.add(infoTextArea);
-		infoTextArea.setText(this.presenter.getAboutText());
+		this.infoTextArea = new JTextArea();
+		this.infoTextArea.setEditable(false);
+		this.infoTextArea.setOpaque(false);
+		infoPanel.add(this.infoTextArea);
 		
 		// ============ SETTINGS PANEL ============
+		this.localeSelector = new JComboBox<>();
 		this.masterPane.addTab("\u2699",
 				new JScrollPane(this.createSettingsPanel()));
 		this.masterPane.setMnemonicAt(5, KeyEvent.VK_S);
 		
 		// ============ FINALIZE CREATION OF VIEW ============
 		this.presenter.postViewInitialize();
+		this.updateText();
 		this.frame.pack();
 		this.frame.setVisible(true);
 	}
@@ -386,7 +412,8 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		{
 			final JPanel roundingPanel = new JPanel();
 			settingsPanel.add(roundingPanel);
-			roundingPanel.setBorder(new TitledBorder("Rounding Settings"));
+			this.localizedTextSetters.put("tv.settings.rounding.title", 
+					txt -> roundingPanel.setBorder(new TitledBorder(txt)));
 			roundingPanel.setLayout(new GridBagLayout());
 			
 			// rounding rule selection
@@ -396,13 +423,17 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 							"Presenter loaded non-standard rounding rule"));
 			this.precision = this.getPresenterPrecision().orElse(6);
 			
-			final JLabel roundingRuleLabel = new JLabel("Rounding Rule:");
+			final JLabel roundingRuleLabel = new JLabel();
+			this.localizedTextSetters.put("tv.settings.rounding.rule", 
+					roundingRuleLabel::setText);
 			roundingPanel.add(roundingRuleLabel, new GridBagBuilder(0, 0)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
 			// sigDigSlider needs to be first so that the rounding-type buttons can
 			// show and hide it
-			final JLabel sliderLabel = new JLabel("Precision:");
+			final JLabel sliderLabel = new JLabel();
+			this.localizedTextSetters.put("tv.settings.rounding.precision", 
+					sliderLabel::setText);
 			sliderLabel.setVisible(
 					this.roundingType != StandardRoundingType.UNCERTAINTY);
 			roundingPanel.add(sliderLabel, new GridBagBuilder(0, 4)
@@ -428,8 +459,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			});
 			
 			// significant digit rounding
-			final JRadioButton fixedPrecision = new JRadioButton(
-					"Fixed Precision");
+			final JRadioButton fixedPrecision = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.rounding.fixed_sigfig", 
+					fixedPrecision::setText);
 			if (this.roundingType == StandardRoundingType.SIGNIFICANT_DIGITS) {
 				fixedPrecision.setSelected(true);
 			}
@@ -444,8 +476,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
 			// decimal place rounding
-			final JRadioButton fixedDecimals = new JRadioButton(
-					"Fixed Decimal Places");
+			final JRadioButton fixedDecimals = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.rounding.fixed_places", 
+					fixedDecimals::setText);
 			if (this.roundingType == StandardRoundingType.DECIMAL_PLACES) {
 				fixedDecimals.setSelected(true);
 			}
@@ -460,8 +493,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
 			// scientific rounding
-			final JRadioButton relativePrecision = new JRadioButton(
-					"Uncertainty-Based Rounding");
+			final JRadioButton relativePrecision = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.rounding.uncertainty", 
+					relativePrecision::setText);
 			if (this.roundingType == StandardRoundingType.UNCERTAINTY) {
 				relativePrecision.setSelected(true);
 			}
@@ -480,8 +514,8 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		{
 			final JPanel prefixRepetitionPanel = new JPanel();
 			settingsPanel.add(prefixRepetitionPanel);
-			prefixRepetitionPanel
-					.setBorder(new TitledBorder("Prefix Repetition Settings"));
+			this.localizedTextSetters.put("tv.settings.repetition.title", 
+					txt -> prefixRepetitionPanel.setBorder(new TitledBorder(txt)));
 			prefixRepetitionPanel.setLayout(new GridBagLayout());
 			
 			final var prefixRule = this.getPresenterPrefixRule()
@@ -491,7 +525,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			// prefix rules
 			final ButtonGroup prefixRuleButtons = new ButtonGroup();
 			
-			final JRadioButton noRepetition = new JRadioButton("No Repetition");
+			final JRadioButton noRepetition = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.repetition.no", 
+					noRepetition::setText);
 			if (prefixRule == DefaultPrefixRepetitionRule.NO_REPETITION) {
 				noRepetition.setSelected(true);
 			}
@@ -504,7 +540,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			prefixRepetitionPanel.add(noRepetition, new GridBagBuilder(0, 0)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JRadioButton noRestriction = new JRadioButton("No Restriction");
+			final JRadioButton noRestriction = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.repetition.any", 
+					noRestriction::setText);
 			if (prefixRule == DefaultPrefixRepetitionRule.NO_RESTRICTION) {
 				noRestriction.setSelected(true);
 			}
@@ -517,8 +555,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			prefixRepetitionPanel.add(noRestriction, new GridBagBuilder(0, 1)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JRadioButton customRepetition = new JRadioButton(
-					"Complex Repetition");
+			final JRadioButton customRepetition = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.repetition.complex", 
+					customRepetition::setText);
 			if (prefixRule == DefaultPrefixRepetitionRule.COMPLEX_REPETITION) {
 				customRepetition.setSelected(true);
 			}
@@ -536,7 +575,8 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		{
 			final JPanel searchingPanel = new JPanel();
 			settingsPanel.add(searchingPanel);
-			searchingPanel.setBorder(new TitledBorder("Search Settings"));
+			this.localizedTextSetters.put("tv.settings.search.title", 
+					txt -> searchingPanel.setBorder(new TitledBorder(txt)));
 			searchingPanel.setLayout(new GridBagLayout());
 			
 			// searching rules
@@ -544,8 +584,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			
 			final var searchRule = this.presenter.getSearchRule();
 			
-			final JRadioButton noPrefixes = new JRadioButton(
-					"Never Include Prefixed Units");
+			final JRadioButton noPrefixes = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.search.no_prefixes", 
+					noPrefixes::setText);
 			noPrefixes.addActionListener(e -> {
 				this.presenter.setSearchRule(PrefixSearchRule.NO_PREFIXES);
 				this.presenter.updateView();
@@ -555,8 +596,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			searchingPanel.add(noPrefixes, new GridBagBuilder(0, 0)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JRadioButton commonPrefixes = new JRadioButton(
-					"Include Common Prefixes");
+			final JRadioButton commonPrefixes = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.search.common_prefixes", 
+					commonPrefixes::setText);
 			commonPrefixes.addActionListener(e -> {
 				this.presenter.setSearchRule(PrefixSearchRule.COMMON_PREFIXES);
 				this.presenter.updateView();
@@ -566,8 +608,9 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			searchingPanel.add(commonPrefixes, new GridBagBuilder(0, 1)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JRadioButton alwaysInclude = new JRadioButton(
-					"Include All Single Prefixes");
+			final JRadioButton alwaysInclude = new JRadioButton();
+			this.localizedTextSetters.put("tv.settings.search.all_prefixes", 
+					alwaysInclude::setText);
 			alwaysInclude.addActionListener(e -> {
 				this.presenter
 						.setSearchRule(this.presenter.getUniversalSearchRule());
@@ -596,30 +639,62 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 			settingsPanel.add(miscPanel);
 			miscPanel.setLayout(new GridBagLayout());
 			
-			final JCheckBox oneWay = new JCheckBox("Convert One Way Only");
+			final JCheckBox oneWay = new JCheckBox();
+			this.localizedTextSetters.put("tv.settings.oneway", oneWay::setText);
 			oneWay.setSelected(this.presenter.oneWayConversionEnabled());
 			oneWay.addItemListener(e -> {
 				this.presenter.setOneWayConversionEnabled(
 						e.getStateChange() == ItemEvent.SELECTED);
 				this.presenter.saveSettings();
 			});
-			miscPanel.add(oneWay, new GridBagBuilder(0, 0)
+			miscPanel.add(oneWay, new GridBagBuilder(0, 0, 2, 1)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JCheckBox showAllVariations = new JCheckBox(
-					"Show Duplicate Units & Prefixes");
+			final JCheckBox showAllVariations = new JCheckBox();
+			this.localizedTextSetters.put("tv.settings.show_duplicate", 
+					showAllVariations::setText);
 			showAllVariations.setSelected(this.presenter.duplicatesShown());
 			showAllVariations.addItemListener(e -> {
 				this.presenter
 						.setShowDuplicates(e.getStateChange() == ItemEvent.SELECTED);
 				this.presenter.saveSettings();
 			});
-			miscPanel.add(showAllVariations, new GridBagBuilder(0, 1)
+			miscPanel.add(showAllVariations, new GridBagBuilder(0, 1, 2, 1)
+					.setAnchor(GridBagConstraints.LINE_START).build());
+			
+			final JCheckBox useDefaultFiles = new JCheckBox();
+			this.localizedTextSetters.put("tv.settings.use_default_files", 
+					useDefaultFiles::setText);
+			useDefaultFiles.setSelected(this.presenter.usingDefaultDatafiles());
+			useDefaultFiles.addItemListener(e -> {
+				this.presenter
+						.setUseDefaultDatafiles(e.getStateChange() == ItemEvent.SELECTED);
+				this.presenter.saveSettings();
+			});
+			miscPanel.add(useDefaultFiles, new GridBagBuilder(0, 2, 2, 1)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 			
-			final JButton unitFileButton = new JButton("Manage Unit Data Files");
+			final JLabel localeLabel = new JLabel();
+			this.localizedTextSetters.put("tv.settings.locale", 
+					localeLabel::setText);
+			miscPanel.add(localeLabel, new GridBagBuilder(0, 3, 1, 1)
+					.setAnchor(GridBagConstraints.LINE_START).build());
+			
+			this.presenter.getAvailableLocales().stream().sorted()
+					.forEachOrdered(this.localeSelector::addItem);
+			this.localeSelector.setSelectedItem(this.presenter.getUserLocale());
+			this.localeSelector.addItemListener(e -> {
+				this.presenter.setUserLocale((String) e.getItem());
+				this.presenter.saveSettings();
+			});
+			miscPanel.add(localeSelector, new GridBagBuilder(1, 3, 1, 1)
+					.setAnchor(GridBagConstraints.LINE_END).build());
+			
+			final JButton unitFileButton = new JButton();
+			this.localizedTextSetters.put("tv.settings.unitfiles.button", 
+					unitFileButton::setText);
 			unitFileButton.setEnabled(false);
-			miscPanel.add(unitFileButton, new GridBagBuilder(0, 2)
+			miscPanel.add(unitFileButton, new GridBagBuilder(0, 4, 2, 1)
 					.setAnchor(GridBagConstraints.LINE_START).build());
 		}
 		
@@ -827,4 +902,13 @@ final class TabbedView implements ExpressionConversionView, UnitConversionView {
 		this.presenter.setNumberDisplayRule(roundingRule);
 		this.presenter.saveSettings();
 	}
+
+	@Override
+	public void updateText() {
+		this.frame.setTitle(this.presenter.getLocalizedText("tv.title")
+				.replace("[v]", ProgramInfo.VERSION.toString()));
+		this.infoTextArea.setText(this.presenter.getAboutText());
+		this.localizedTextSetters.forEach((id, action) ->
+			action.accept(this.presenter.getLocalizedText(id)));
+	}
 }
diff --git a/src/main/java/sevenUnitsGUI/View.java b/src/main/java/sevenUnitsGUI/View.java
index 7dd0c44..4140992 100644
--- a/src/main/java/sevenUnitsGUI/View.java
+++ b/src/main/java/sevenUnitsGUI/View.java
@@ -112,4 +112,11 @@ public interface View {
 	 */
 	void showUnit(NameSymbol name, String definition, String dimensionName,
 			UnitType type);
+	
+	/**
+	 * Updates the view's text to reflect the presenter's locale.
+	 * 
+	 * This method <b>must not</b> call {@link Presenter#setUserLocale(String)}.
+	 */
+	void updateText();
 }
diff --git a/src/main/java/sevenUnitsGUI/ViewBot.java b/src/main/java/sevenUnitsGUI/ViewBot.java
index e6593fb..8fff46d 100644
--- a/src/main/java/sevenUnitsGUI/ViewBot.java
+++ b/src/main/java/sevenUnitsGUI/ViewBot.java
@@ -505,4 +505,9 @@ public final class ViewBot
 	public List<UnitViewingRecord> unitViewList() {
 		return Collections.unmodifiableList(this.unitViewingRecords);
 	}
+
+	@Override
+	public void updateText() {
+		// do nothing, since ViewBot is not localized
+	}
 }
diff --git a/src/main/resources/about.txt b/src/main/resources/about/en.txt
index 4c33f8c..068f922 100644
--- a/src/main/resources/about.txt
+++ b/src/main/resources/about/en.txt
@@ -15,7 +15,7 @@ Unit/Prefix/Dimension Statistics:
 
 Copyright Notice:
 
-Unit Converter Copyright (C) 2018-2024 Adrien Hopkins
+Unit Converter Copyright (C) 2018-2025 Adrien Hopkins
 This program comes with ABSOLUTELY NO WARRANTY;
 for details read the LICENSE file, section 15
 
diff --git a/src/main/resources/about/fr.txt b/src/main/resources/about/fr.txt
new file mode 100644
index 0000000..d8d82aa
--- /dev/null
+++ b/src/main/resources/about/fr.txt
@@ -0,0 +1,24 @@
+À propos de 7Unités version [VERSION]
+
+7Unités est une programme pour convertir les unités avec plusieurs fonctions,
+inspiré par GNU Units (https://www.gnu.org/software/units/).
+Vous pouvez l’utiliser pour convertir des unités, mais vous pouvez aussi
+l’utiliser comme calculatrice, computer et convertir des expressions
+comme "10 m/s + (25^2 - 5^2) mi/hr".
+
+Ce logiciel est par Adrien Hopkins <adrien.p.hopkins@gmail.com>.
+
+Statistiques d’unités, préfixes et dimensions:
+
+[LOADSTATS]
+
+Copyright Notice:
+
+Unit Converter Copyright (C) 2018-2025 Adrien Hopkins
+This program comes with ABSOLUTELY NO WARRANTY;
+for details read the LICENSE file, section 15
+
+This is free software, and you are welcome to redistribute
+it under certain conditions; for details go to
+<https://www.gnu.org/licenses/quick-guide-gplv3.html>
+or read the LICENSE file.
diff --git a/src/main/resources/locales/en.txt b/src/main/resources/locales/en.txt
new file mode 100644
index 0000000..666e363
--- /dev/null
+++ b/src/main/resources/locales/en.txt
@@ -0,0 +1,31 @@
+tv.title=7Units [v]
+tv.convert_units.title=Convert Units
+tv.convert_units.value_prompt=Value to convert:
+tv.convert_units.convert_btn=Convert
+tv.convert_expressions.title=Convert Unit Expressions
+tv.convert_expressions.from=From
+tv.convert_expressions.to=To
+tv.convert_expressions.convert_btn=Convert
+tv.convert_expressions.output=Output
+tv.unit_viewer.title=Unit Viewer
+tv.prefix_viewer.title=Prefix Viewer
+tv.settings.rounding.title=Rounding Settings
+tv.settings.rounding.rule=Rounding Rule:
+tv.settings.rounding.precision=Precision:
+tv.settings.rounding.fixed_sigfig=Fixed Precision
+tv.settings.rounding.fixed_places=Fixed Decimal Places
+tv.settings.rounding.uncertainty=Uncertainty-based Rounding
+tv.settings.repetition.title=Prefix Repetition Settings
+tv.settings.repetition.no=No Repetition
+tv.settings.repetition.any=No Restriction
+tv.settings.repetition.complex=Complex Repetition
+tv.settings.search.title=Search Settings
+tv.settings.search.no_prefixes=Never Include Prefixed Units
+tv.settings.search.common_prefixes=Include Common Prefixes
+tv.settings.search.all_prefixes=Include All Single Prefixes
+tv.settings.oneway=Convert One Way Only
+tv.settings.show_duplicate=Show Duplicate Units & Prefixes
+tv.settings.use_default_files=Use Default Datafiles
+tv.settings.locale=🌐 Locale:
+tv.settings.unitfiles.button=Manage Unit Data Files
+load_stat_msg=Successfully loaded [u] unique units with [un] names ([b] base units), [p] unique prefixes with [pn] names, [s] unit sets, and [d] named dimensions.
diff --git a/src/main/resources/locales/fr.txt b/src/main/resources/locales/fr.txt
new file mode 100644
index 0000000..3fef030
--- /dev/null
+++ b/src/main/resources/locales/fr.txt
@@ -0,0 +1,31 @@
+tv.title=7Unités [v]
+tv.convert_units.title=Convertir Unités
+tv.convert_units.value_prompt=Valueur à convertir:
+tv.convert_units.convert_btn=Convertir
+tv.convert_expressions.title=Convertir Expressions
+tv.convert_expressions.from=De
+tv.convert_expressions.to=À
+tv.convert_expressions.convert_btn=Convertir
+tv.convert_expressions.output=Résultat
+tv.unit_viewer.title=Unités
+tv.prefix_viewer.title=Préfixes
+tv.settings.rounding.title=Préférences d’Arrondissement
+tv.settings.rounding.rule=Règle d’Arrondissement
+tv.settings.rounding.precision=Précision:
+tv.settings.rounding.fixed_sigfig=Precision fixe
+tv.settings.rounding.fixed_places=Chiffres fixe
+tv.settings.rounding.uncertainty=Arrondissement d’Incertitude
+tv.settings.repetition.title=Préférences de répetition de préfixes
+tv.settings.repetition.no=Non répetition
+tv.settings.repetition.any=Tout répetition
+tv.settings.repetition.complex=Répetition Complexe
+tv.settings.search.title=Préférences de Recherche
+tv.settings.search.no_prefixes=Jamais inclure unités préfixés
+tv.settings.search.common_prefixes=Inclure préfixes fréquents
+tv.settings.search.all_prefixes=Inclure tous préfixes seuls
+tv.settings.oneway=Convertir Seulement en un Direction
+tv.settings.show_duplicate=Montrer unités et préfixes doubles
+tv.settings.use_default_files=Utilise donées par défaut
+tv.settings.locale=🌐 Locale:
+tv.settings.unitfiles.button=Gérer donées d’unités
+load_stat_msg=Chargé [u] unités uniques avec [un] noms ([b] unités bases), [p] préfixes uniques avec [pn] noms, [s] collections d’unités, et [d] dimensions nomées.
diff --git a/src/test/java/sevenUnitsGUI/I18nTest.java b/src/test/java/sevenUnitsGUI/I18nTest.java
new file mode 100644
index 0000000..73bd727
--- /dev/null
+++ b/src/test/java/sevenUnitsGUI/I18nTest.java
@@ -0,0 +1,21 @@
+package sevenUnitsGUI;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import org.junit.jupiter.api.Test;
+
+class I18nTest {
+
+	/**
+	 * Tests that the default locale exists.
+	 * 
+	 * Currently this test fails.
+	 */
+	@Test
+	void testDefaultLocale() {
+		Presenter p = new Presenter(new ViewBot());
+		assertNotNull(p.locales.get(Presenter.DEFAULT_LOCALE),
+				"Default locale does not exist.");
+	}
+
+}
diff --git a/src/test/java/sevenUnitsGUI/PresenterTest.java b/src/test/java/sevenUnitsGUI/PresenterTest.java
index 9e25a08..8b16365 100644
--- a/src/test/java/sevenUnitsGUI/PresenterTest.java
+++ b/src/test/java/sevenUnitsGUI/PresenterTest.java
@@ -17,6 +17,7 @@
 package sevenUnitsGUI;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
@@ -154,6 +155,23 @@ public final class PresenterTest {
 				expectedOutput.getValue().toString(false, RoundingMode.HALF_EVEN));
 		assertEquals(List.of(expectedUC), viewBot.unitConversionList());
 	}
+	
+	/**
+	 * Ensures that the default unitfile can be disabled.
+	 * 
+	 * @since v1.0.0
+	 * @since 2025-02-23
+	 */
+	@Test
+	void testDisableDefault() {
+		final var viewBot = new ViewBot();
+		final var presenter = new Presenter(viewBot);
+		assumeTrue(presenter.database.containsUnitName("joule"),
+				"Attempted to test disabling default on unit not in default file.");
+		presenter.setUseDefaultDatafiles(false);
+		assertFalse(presenter.database.containsUnitName("joule"),
+				"Presenter disabled default datafiles, but still contains the joule.");
+	}
 
 	/**
 	 * Tests that duplicate units are successfully removed, if that is asked for