Comment 1 for bug 249989

The user interface is determined by what you have downloaded (there isn't a single version where you select with an environment variable) and the content is determined by Tools->Options->Content->Languages (where you can select multiple language and it depends on the website what it will show you).

This is not a bug in my opinion, unless you want Firefox to be delivered in a single version for all languages (in which case nobody should complain that it's much larger than what it is now).