Allows the Minecraft dedicated server to see localized (TranslatableText) messages. Activate in your ModInitializer only on the server side

LanguageHack.kt

package net.logandark.languagehack

import com.google.gson.Gson
import com.google.gson.JsonObject
import net.fabricmc.loader.api.FabricLoader
import net.logandark.languagehack.mixin.MixinLanguage
import net.minecraft.util.JsonHelper
import net.minecraft.util.Language
import org.apache.logging.log4j.core.util.Closer
import java.io.InputStreamReader
import java.nio.charset.StandardCharsets

object LanguageHack {
	fun activate(modid: String) {
		val language = Language.getInstance() as MixinLanguage
		val inputStream = Files.newInputStream(
			FabricLoader.getInstance()
				.getModContainer(modid).get()
				.getPath("assets/$modid/lang/en_us.json")
		)

		try {
			val jsonObject = Gson().fromJson(
				InputStreamReader(inputStream, StandardCharsets.UTF_8),
				JsonObject::class.java
			)

			jsonObject.entrySet().forEach { entry ->
				val string = language.field_11489
					.matcher(JsonHelper.asString(entry.value, entry.key))
					.replaceAll("%$1s")
				language.translations[entry.key] = string
			}
		} finally {
			Closer.closeSilently(inputStream)
		}
	}
}

MixinLanguage.java

package net.logandark.languagehack.mixin;

import net.minecraft.util.Language;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.gen.Accessor;

import java.util.Map;
import java.util.regex.Pattern;

@Mixin(Language.class)
public interface MixinLanguage {
	@Accessor
	Map<String, String> getTranslations();

	@Accessor
	Pattern getField_11489();
}

usage.kt

package net.logandark.example

import net.fabricmc.api.EnvType
import net.fabricmc.api.ModInitializer
import net.fabricmc.loader.api.FabricLoader
import net.logandark.languagehack.LanguageHack

@Suppress("unused")
object ExampleMod : ModInitializer {
	const val modid = "example-mod"

	override fun onInitialize() {
		// Activate the language hack for localizations on server.
		if (FabricLoader.getInstance().environmentType == EnvType.SERVER)
			LanguageHack.activate(modid)
	}
}