I think such a site can be viable, can include with its scope all sorts of questions about specific languages or about linguistics (with specific details about scope TBD). I think all the Q&A can be in one category, with tags for specific languages, for language families, for subfamilies, and for areas of study within linguistics. It can include questions of the sort found on the linguistics, English, and ESL Stack Exchange sites, along with questions about other languages like the aforementioned English ones. The main problem I can think of with this is the very varied expertise levels, ESL questions alongside professional linguists'. We see this at Judaism SE and it leads to a few problems. (a) Tyros don't tag questions correctly. They simply don't know that the question is a morphology question. (b) People answer at the wrong level, including too much jargon or too little info for a tyro. (c) It's hard to find questions at the level you want unless there's a tag (or, here, category) for level, which there can't really be because it's a spectrum rather than a binary assignment.