Artificial languages with native speakers should be on topic (Esperanto questions have already survived on the site- i.e. they were asked and answered without too much threat of closure). I personally think they are on topic because anything with native speakers is easily in the domain of science and has objective answers.
Artificial languages without native speakers should be off topic. (Although this could admin Volapuk and Klingon which each had/have a single native speaker) The non-natural languages are partially in the realm of art. When these questions do pop up, they should be referred to writers SE, sci-fi SE or the conlang SE proposal. On writers and sci-fi, about a half dozen conlang questions have been asked, answered and haven't been closed.
This is actually a difficult rule to write because there is a lot of grey area.
Natural languages with native speakers are on topic. Fictional, constructed and auxiliary languages without native speakers are off topic. This should not discourage anyone from asking about language formalisms, such as context free grammars. Consider asking questions about fictional languages on Writers SE, Science-Fiction Fantasy SE or the Conlang SE proposal.
This would pretty much rules out Lojban, Na'vi, Elvish, Shyriiwook, toki pona, Laadan, languages invented by a single person as a hobby, New Speak, almost all auxiliary language proposals except Esperanto. There are single reports of native speakers of Volapuk, Klingon and a handful of cases of native speakers of previously dead languages-- Latin, Sanskrit. Other than that, the rule at least would have little grey area.
I don't know about non-recreational medical conlangs, like Bliss Symbols or those communication systems invented for the deal and blind or otherwise disabled, which can have the characteristics of a constructed language (i.e. go beyond being an encoding of English). I suppose the community can figure out what to do with those questions if they happen.