The site is for "professional linguists and others with an interest in linguistic research and theory," according to the FAQ. Presumably "professional linguists (etc.)" are interested in topics other than theoretical linguistics, like etymology and translation (and knitting and computer programming...) but the unifying trait of that group is theoretical linguistics Q&A. The "if not us, who?" argument is unavailing, both in principle and given the existence of this proposal on Area 51, which can absorb most if not all of the off-topic questions I have seen proposed.
There are theoretical aspects of "language acquisition, second language acquisition, computational linguistics, language development, comparative linguistics, [and] phonetics" (from Alenanno's answer). I don't know why it is believed that anyone has proposed to exclude questions on these topics.
I wrote a fairly extensive answer to this question, which also touches on these issues. I think that it was rightly decided there that questions don't need to be "research level" in order to be welcome here, but I what we are discussing now is whether they need to have some inherent connection to (a certain) linguistic theory.
For concreteness, the only questions which I judged to be clearly off topic so far have been:
This one on the "linguistics wars" was on topic but poorly framed.
For concreteness, can others who weigh in say why they might consider the above questions on topic, and/or mention other questions whose topicality they wish to discuss?