Refactor language version behavior in FSC

[psfinaki]

@KevinRansom Yep, thanks for the point, I think I will address that separately in a followup. That thing is just ridiculous - the function is called "setLanguageVersion" but actually it sometimes sets the version and sometimes gets the versions. That's like against all the coding principles, so I'll better do a small refactoring and testing around that.

Originally posted by @psfinaki in #13702 (comment)