You can generate translation entries from existing language files and classes that support translation keys.

Firstly, create a class that extends FabricLanguageProvider and implement the base methods like so:

private static class MyModEnglishLangProvider extends FabricLanguageProvider {
	private MyModEnglishLangProvider(FabricDataOutput dataGenerator, CompletableFuture<RegistryWrapper.WrapperLookup> registryLookup) {
                // Specifying en_us is optional, by default it is en_us.
		super(dataGenerator, "en_us", registryLookup);
	}
 
	@Override
	public void generateTranslations(RegistryWrapper.WrapperLookup registryLookup, TranslationBuilder translationBuilder) {
 
	}
}

Lets add some translation entries:

@Override
public void generateTranslations(RegistryWrapper.WrapperLookup registryLookup, TranslationBuilder translationBuilder) {
	translationBuilder.add(SIMPLE_ITEM, "Simple Item");
	translationBuilder.add(SIMPLE_BLOCK, "Simple Block");
	translationBuilder.add(SIMPLE_ITEM_GROUP, "Simple Item Group");
 
	// Load an existing language file.
	try {
		Path existingFilePath = dataGenerator.getModContainer().findPath("assets/mymod/lang/en_us.existing.json").get();
		translationBuilder.add(existingFilePath);
	} catch (Exception e) {
		throw new RuntimeException("Failed to add existing language file!", e);
	}
}

Now, we will need to add the provider to our data generator in the onInitializeDataGenerator in your entrypoint class like so:

public class DataGeneration implements DataGeneratorEntrypoint {
    @Override
    public void onInitializeDataGenerator(FabricDataGenerator fabricDataGenerator) {
        Pack pack = fabricDataGenerator.createPack();
        pack.addProvider(MyModEnglishLangProvider::new);
    }
}

These are the game objects that have a translation key for you to translate:

• Item
• Block
• ItemGroup
• EntityType<T>
• Enchantment
• EntityAttribute
• StatType<T>
• StatusEffect
• Identifier