Proposal: Constructed languages
What I mean is would questions about synthesizing my own languages be on topic, or only questions about existing ones?
I think it should, but it should be up to all of us to decide on it.
I should think that questions concerning your own, or anyone else's invented language would be on topic here. That is, whether you have questions about how to explain some portion of your grammar, or how a particular aspect of grammar can be modified to work within your language. Or how did you solve this issue in your own language.
Questions about existing invented languages, especially obscure ones, but also more well known ones ought to be on topic, such as there used to be a page about Teonaht - - where can I find out about it now? I agree with Sparksbet that the Constructed Langauges StackEx should not become a how do I say "go pound sand" in Esperanto or Klingon or Sindarin. The big names have their own forums & often websites or other resources that can be consulted.
Natural Language Questions:
Invariably, language invention forums will have to deal with questions of how does this aspect of grammar work in a natural language. We need to understand these things before we can implement them in a constructed language. This Stack shouldn't become an English Grammar 201 forum, but we do need to have some flexibility in describing what goes on in natural languages.
Odds & Ends:
Bookkeeping questions like how do I display or showcase my language; what resources are there available for keeping track of things should also be on topic.
International Auxiliary Languages:
I definitely think that the politics and proselytization of auxiliary invented languages ought to be 100% verboten. Preaching the qualities of one's preferred IAL over the qualities of another IAL have no place in a Q&A forum like StackEx. Of course, questions regarding the design, making and testing of an invented IAL ought to be well within the scope of this Stack.
If anything, questions about existing constructed languages should be a rather small proportion of the questions on this stackexchange -- if a particular existing conlang is popular enough, it should have its own resources and perhaps its own stackexchange (like Esperanto does, if I remember correctly).