I would appreciate if someone could clarify what the policy and etiquette are about using languages other than English on MSE, or point me to an authoritative resource on the subject.
What prompted the question is that I recently flagged a non-English answer to an English question with a comment "thought answers were supposed to be in English", but the flag was declined by reason of "no such requirement exists on Math SE, though English is preferred as a *lingua franca*".
[ EDIT ] $\;$ Thanks to all who commented. Below is a summary of the main points made.
MSE does not have an official policy on questions and answers in languages other than English. Previous discussions on this subject indicated openness to consider other languages and help with addressing and/or translating such posts in English (1, 2), but opinions were divided, with no clear consensus.
SE does have the policy of not allowing posts in other languages, posted under the heading "official policy" in a single-answer faq: "unless you're posting on a language-related site (e.g. French Language) or a site where all questions are expected to be in a different language (e.g. Stack Overflow in Spanish), yes, all posts are expected to be in English".
Moreover, a comment under the answer clarifies that "yes, all SE sites are English-only by default, with the exception of language sites [...] and localized sites".
The "what should I do if someone else makes a post that is not in English" paragraph makes no mention of moderator intervention for such policy violations, but rather advises users to do the policing via downvotes and/or VLQ flags.
The only MSE mod who engaged in the conversation confirmed that MSE does not have a language policy of its own, yet questions "the assumption that the global policy applies here (which is not entirely clear...", and is of the opinion that, regardless, violations of the language policy do not fall under mod's radar, citing the "what should I do if" part from the SE post.
For the record, my position is that SE policies which are not specifically overridden by a local MSE policy should be binding for MSE, that resolving policy violations is an attribution of mods, not users, and that using downvotes and/or VLQ flags for purposes other than those they were meant for is a misuse. That said, I consider the question closed and have no intention to pursue it further.