Mods are glorified janitors - we try to do our best to cleanup where things are drastically/dramatically need our intervention. Examples - spam, bad language, etc etc.
However, we - like you - are users of the site who can edit and fix-up questions and answers with gramatical errors, typos, poor headings etc.
Language issues fall in the latter category IMHO. Anyone can translate. Anyone can also use google-translate or other translation tools.
Whilst there are a few mods with multiple language skills, most of us are native english speakers and at best would use google-translate.
Thus - its not for Mods to edit these sorts of questions. This task is for all of us - all editors of Ask Ubuntu. Hence, flags shouldnt really be used for this editor task.
If you dont feel comfortable doing the translation - just drop a comment, reminding the OP of the FAQ and pointing out the use of English. Other members of AU who do speak the OP's language will do the translation.
TL;DR: Do not flag language issues. Translate them yourself.