Add Locale conversion format configuration property

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedInvocationContext.java

@@ -72,7 +72,7 @@ private void storeParameterInfo(ExtensionContext context) {
 		ParameterDeclarations declarations = this.declarationContext.getResolverFacade().getIndexedParameterDeclarations();
 		ClassLoader classLoader = getClassLoader(this.declarationContext.getTestClass());
 		Object[] arguments = this.arguments.getConsumedPayloads();
-		ArgumentsAccessor accessor = DefaultArgumentsAccessor.create(invocationIndex, classLoader, arguments);
+		ArgumentsAccessor accessor = DefaultArgumentsAccessor.create(context, invocationIndex, classLoader, arguments);
 		new DefaultParameterInfo(declarations, accessor).store(context);
 	}
 

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ResolverFacade.java

@@ -425,7 +425,7 @@ private static Converter createConverter(ParameterDeclaration declaration, Exten
 					.map(clazz -> ParameterizedTestSpiInstantiator.instantiate(ArgumentConverter.class, clazz, extensionContext))
 					.map(converter -> AnnotationConsumerInitializer.initialize(declaration.getAnnotatedElement(), converter))
 					.map(Converter::new)
-					.orElse(Converter.DEFAULT);
+					.orElse(Converter.defaultConverter(extensionContext));
 		} // @formatter:on
 		catch (Exception ex) {
 			throw parameterResolutionException("Error creating ArgumentConverter", ex, declaration.getParameterIndex());
@@ -467,10 +467,12 @@ Object resolve(FieldContext fieldContext, ExtensionContext extensionContext, Eva
 
 	private static class Converter implements Resolver {
 
-		private static final Converter DEFAULT = new Converter(DefaultArgumentConverter.INSTANCE);
-
 		private final ArgumentConverter argumentConverter;
 
+		private static Converter defaultConverter(ExtensionContext context) {
+			return new Converter(new DefaultArgumentConverter(context));
+		}
+
 		Converter(ArgumentConverter argumentConverter) {
 			this.argumentConverter = argumentConverter;
 		}

junit-jupiter-params/src/main/java/org/junit/jupiter/params/aggregator/DefaultArgumentsAccessor.java

@@ -19,6 +19,7 @@
 import java.util.function.BiFunction;
 
 import org.apiguardian.api.API;
+import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.params.converter.DefaultArgumentConverter;
 import org.junit.platform.commons.util.ClassUtils;
 import org.junit.platform.commons.util.Preconditions;
@@ -40,10 +41,11 @@ public class DefaultArgumentsAccessor implements ArgumentsAccessor {
 	private final Object[] arguments;
 	private final BiFunction<Object, Class<?>, Object> converter;
 
-	public static DefaultArgumentsAccessor create(int invocationIndex, ClassLoader classLoader, Object[] arguments) {
+	public static DefaultArgumentsAccessor create(ExtensionContext context, int invocationIndex,
+			ClassLoader classLoader, Object[] arguments) {
 		Preconditions.notNull(classLoader, "ClassLoader must not be null");
 
-		BiFunction<Object, Class<?>, Object> converter = (source, targetType) -> DefaultArgumentConverter.INSTANCE //
+		BiFunction<Object, Class<?>, Object> converter = (source, targetType) -> new DefaultArgumentConverter(context) //
 				.convert(source, targetType, classLoader);
 		return new DefaultArgumentsAccessor(converter, invocationIndex, arguments);
 	}

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/DefaultArgumentConverter.java

@@ -21,8 +21,10 @@
 import java.util.Currency;
 import java.util.Locale;
 import java.util.UUID;
+import java.util.function.Function;
 
 import org.apiguardian.api.API;
+import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.ParameterContext;
 import org.junit.jupiter.params.support.FieldContext;
 import org.junit.platform.commons.support.conversion.ConversionException;
@@ -50,10 +52,16 @@
 @API(status = INTERNAL, since = "5.0")
 public class DefaultArgumentConverter implements ArgumentConverter {
 
-	public static final DefaultArgumentConverter INSTANCE = new DefaultArgumentConverter();
+	private static final String DEFAULT_LOCALE_CONVERSION_FORMAT_PROPERTY_NAME = "junit.jupiter.params.arguments.conversion.locale.format";
 
-	private DefaultArgumentConverter() {
-		// nothing to initialize
+	private static final Function<String, LocaleConversionFormat> CONFIGURATION_TRANSFORMER = value -> LocaleConversionFormat.valueOf(
+		value.trim().toUpperCase(Locale.ROOT));
+
+	private final LocaleConversionFormat format;
+
+	public DefaultArgumentConverter(ExtensionContext context) {
+		this.format = context.getConfigurationParameter(DEFAULT_LOCALE_CONVERSION_FORMAT_PROPERTY_NAME,
+			CONFIGURATION_TRANSFORMER).orElse(LocaleConversionFormat.BCP_47);
 	}
 
 	@Override
@@ -84,6 +92,10 @@ public final Object convert(Object source, Class<?> targetType, ClassLoader clas
 		}
 
 		if (source instanceof String) {
+			if (targetType == Locale.class && format == LocaleConversionFormat.BCP_47) {
+				return Locale.forLanguageTag((String) source);
+			}
+
 			try {
 				return convert((String) source, targetType, classLoader);
 			}

junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/LocaleConversionFormat.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.params.converter;
+
+import static org.apiguardian.api.API.Status.INTERNAL;
+
+import org.apiguardian.api.API;
+
+/**
+ * Enumeration of {@link java.util.Locale} conversion formats.
+ *
+ * @since 5.13
+ */
+@API(status = INTERNAL, since = "5.13")
+public enum LocaleConversionFormat {
+
+	/**
+	 * The ISO 639 alpha-2 or alpha-3 language code format.
+	 *
+	 * @see java.util.Locale#Locale(String)
+	 */
+	ISO_639,
+
+	/**
+	 * The IETF BCP 47 language tag format.
+	 *
+	 * @see java.util.Locale#forLanguageTag(String)
+	 */
+	BCP_47
+
+}

jupiter-tests/src/test/java/org/junit/jupiter/params/aggregator/DefaultArgumentsAccessorTests.java

@@ -16,10 +16,12 @@
 import static org.junit.jupiter.api.Assertions.assertIterableEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.mock;
 
 import java.util.Arrays;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.platform.commons.PreconditionViolationException;
 
 /**
@@ -164,8 +166,9 @@ void size() {
 	}
 
 	private static DefaultArgumentsAccessor defaultArgumentsAccessor(int invocationIndex, Object... arguments) {
+		var context = mock(ExtensionContext.class);
 		var classLoader = DefaultArgumentsAccessorTests.class.getClassLoader();
-		return DefaultArgumentsAccessor.create(invocationIndex, classLoader, arguments);
+		return DefaultArgumentsAccessor.create(context, invocationIndex, classLoader, arguments);
 	}
 
 	@SuppressWarnings("unused")

jupiter-tests/src/test/java/org/junit/jupiter/params/converter/DefaultArgumentConverterTests.java

@@ -16,11 +16,13 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.doThrow;
+import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.verify;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 import org.junit.platform.commons.support.ReflectionSupport;
@@ -35,7 +37,8 @@
  */
 class DefaultArgumentConverterTests {
 
-	private final DefaultArgumentConverter underTest = spy(DefaultArgumentConverter.INSTANCE);
+	private final ExtensionContext context = mock();
+	private final DefaultArgumentConverter underTest = spy(new DefaultArgumentConverter(context));
 
 	@Test
 	void isAwareOfNull() {

jupiter-tests/src/test/kotlin/org/junit/jupiter/params/aggregator/ArgumentsAccessorKotlinTests.kt

@@ -13,6 +13,8 @@ import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
+import org.junit.jupiter.api.extension.ExtensionContext
+import org.mockito.Mockito.mock
 
 /**
  * Unit tests for using [ArgumentsAccessor] from Kotlin.
@@ -52,8 +54,9 @@ class ArgumentsAccessorKotlinTests {
         invocationIndex: Int,
         vararg arguments: Any
     ): DefaultArgumentsAccessor {
+        val context = mock(ExtensionContext::class.java)
         val classLoader = ArgumentsAccessorKotlinTests::class.java.classLoader
-        return DefaultArgumentsAccessor.create(invocationIndex, classLoader, arguments)
+        return DefaultArgumentsAccessor.create(context, invocationIndex, classLoader, arguments)
     }
 
     fun foo() {