That is the only three languages supported by the game at this time. It is driven from the game settings on what you play in. During runtime it is stored as a CVar. That is "Language.2" string. This matches the variable name in the setting.txt in the launcher application folder.
Code:
PlayersLanguage = GetCVar("Language.2")
That said you can expand to other languages using your own string lists but the manifest flip won't help you. You will have to track and load all your strings yourself for other languages. Before knowing about the manifest option, I was doing just that for my add-on.
The $(language) is just a variable swapped out. You are free to add text it i.e.
localization/AddonName$(language)IjustLikeLongNames.lua
localization/AddonNameDEIjustLikeLongNames.lua
--Halja