If someone asks why a film was produced in English instead of Japanese (for example only), is that considered "behind the scenes" or is it off topic?
I don't see why it should be off topic, it seems to fit firmly in the behind the scenes area. As long as the work is SFF-nal you shouldn't have a problem.
That said these types of questions don't seem like they would be particularly interesting as I imagine 99% of the time the answer would be "because it was for an English speaking audience not a Japanese one" or "because the cast was English" to use your example. So, I'd expect the question would get some downvotes unless you could explain why it would have made more sense to produce it in Japanese.