I read that some lyrics questions are on topic and some are not. Is it the same for subtitles? This could come up if someone watches a movie in a language other than English, and has a question about an English subtitle. Is this another "it depends" situation?
I'll go with Mitch's sentiment:
As with anything, it's on topic if it's on topic already by other principles. If you question is "Is this subtitle 'blah blah blah' correct for 'bleuh bleuh bleuh'?" with no link and no explanation of why you think it is wrong, and no context of the other language and the question not being primarily about the English, then no it is not on topic. If the question is about the idiomaticity or register or grammatical structure of the English result (you should specify the details desired and the details of the nuance of the original) then I'd go in the direction of 'yes'.
We have more than enough rules or conventions about what is on-topic. It shouldn't matter what the source material is; if the question is on-topic, it's on-topic. It might help to have a link to the right point in a video clip, but since video can't be searched and isn't particularly accessible, the question should contain everything it needs in plain text.
If the text of the question makes it clear it's a good question, that's fine. If the text of the question indicates it's a bad question, it can be dealt with accordingly.
With regard to the specifics of what prompted the question, translation issues can be on-topic, but it's not easy to get right. Plot inconsistencies don't relate to the nuts and bolts of the English language and are more-or-less guaranteed to be off-topic.