I'm looking for Minecraft language files to a resourcepack, but I only found the English US one. Where are the another ones?
See this question here.
What you can do is create language support in resource packs as you can see in maps like The Heist.
In the resource pack go to assets/minecraft/lang folder. Create files like EN_US.lang with content like:
language.name=English language.region=US language.code=en_US container.dropper=Dopfer container.furnace=New funace container.enchant=Book stealer container.crafting= deathScreen.respawn=Restart! deathScreen.title=Game Over
You cannot add new languages without mods. You have to change existing ones.