RevTools - lang.getLanguage
From Riftui Wiki
Use this function to get the language of the client. You might ask why to use this function instead of Inspect.System.Language() which is a good question. The answer to this is that if you use this function instead you'll be able to override the system language by using revTools.lang.setLanguage(). This will even work a load time so you can easily test different language files which are loaded at addon load time!
local language = revTools.lang.getLanguage()