So far, I've only seen questions on English-specific books. Is this a language restriction?
The language of the site is English, so the audience are people who speak English. This strongly biases the site towards books written in English (or at least for which translations are available).
Furthermore, I think that there is a larger proportion of SF published in English than with other genres. This is an informal impression from comparing the amount of SF not translated from English relative to the total amount of SF, to the relative amount for other genres and non-genre modern literature, in France at least.
So it's natural that the bulk of the SF we discuss was written in English or at least translated into English. However, this is no obligation, and I do recall a couple of story identification questions where the asker had read a book in another language and wasn't sure there was an English translation. I think we've also had a couple of questions about non-English writers, not all of whose books had been translated into English.
You can ask questions about non-English books. You're less likely to reach someone who can answer, but there's no prohibition against such questions.