[1.20.1] Fix overriding vanilla translations (Backport of #4187)

fabric-resource-loader-v0/src/main/java/net/fabricmc/fabric/mixin/resource/loader/server/LanguageMixin.java

@@ -36,6 +36,8 @@
 import net.minecraft.util.Language;
 
 import net.fabricmc.fabric.impl.resource.loader.ServerLanguageUtil;
+import net.fabricmc.loader.api.FabricLoader;
+import net.fabricmc.loader.api.ModContainer;
 
 @Mixin(Language.class)
 class LanguageMixin {
@@ -54,6 +56,18 @@ private static ImmutableMap<String, String> create(ImmutableMap.Builder<String,
 		return ImmutableMap.copyOf(map);
 	}
 
+	@Redirect(method = "load(Ljava/util/function/BiConsumer;Ljava/lang/String;)V", at = @At(value = "INVOKE", target = "Ljava/lang/Class;getResourceAsStream(Ljava/lang/String;)Ljava/io/InputStream;"))
+	private static InputStream readCorrectVanillaResource(Class instance, String path) throws IOException {
+		ModContainer mod = FabricLoader.getInstance().getModContainer("minecraft").orElseThrow();
+		Path langPath = mod.findPath(path).orElse(null);
+
+		if (langPath == null) {
+			throw new IOException("Could not read %s from minecraft ModContainer".formatted(path));
+		} else {
+			return Files.newInputStream(langPath);
+		}
+	}
+
 	private static void loadFromPath(Path path, BiConsumer<String, String> entryConsumer) {
 		try (InputStream stream = Files.newInputStream(path)) {
 			LOGGER.debug("Loading translations from {}", path);

fabric-resource-loader-v0/src/testmod/java/net/fabricmc/fabric/test/resource/loader/LanguageTestMod.java

@@ -27,6 +27,8 @@ public void onInitializeServer() {
 	}
 
 	private static void testTranslationLoaded() {
+		testTranslationLoaded("item.minecraft.potato", "Potato"); // Test that vanilla translation loads
+		testTranslationLoaded("text.fabric-resource-loader-v0-testmod.server.lang.override", "Vanilla override test");
 		testTranslationLoaded("pack.source.fabricmod", "Fabric mod");
 		testTranslationLoaded("text.fabric-resource-loader-v0-testmod.server.lang.test0", "Test from fabric-resource-loader-v0-testmod");
 		testTranslationLoaded("text.fabric-resource-loader-v0-testmod.server.lang.test1", "Test from fabric-resource-loader-v0-testmod-test1");

fabric-resource-loader-v0/src/testmod/resources/assets/minecraft/lang/en_us.json

@@ -0,0 +1,3 @@
+{
+  "text.fabric-resource-loader-v0-testmod.server.lang.override": "Vanilla override test"
+}