Is EL&U the right place to ask linguistic questions that do not pertain to any specific language? E.g. questions on grammar that is shared by most every language.
If the question pertains equally to English and any other language, but if it can be answered by referring mainly to English, I'd say it is fine. The question should also invite a response that is mainly about English; if it does not, it is borderline. For a good question of this sort I am thinking of something like this:
In many languages, the line between different kinds of modality, such as obligation and probability, tends to shift: modal verbs change in meaning over time, and new ones are born. Is there any theory that explains why this happens? I believe that English "can" is used to grant permission now more often than it used to be.
A good answer would use mainly English examples to elucidate the general phenomenon. If we cannot use universal patterns and comparisons with other languages, how can we hope to deal with English efficiently? We should only banish "interdisciplinary" knowledge if it is fully off topic. Of course this question could go the wrong way if the answers are not about English at all; but I feel that this is a risk we must accept.