This question is similar to this one, but wants to focus on a different aspect of the matter. Personally, I think that both English and Italian questions should be accepted, but I have seen cases in which a question is in English and one of the answers is in Italian (for the sake of this question, also the opposite situation counts).
What should we do when this happens? Is this accepted behavior or is it something that should be flagged and reported?
I think we should leave things right as they are. That is, if someone answers in the other languages, then it's up to the owner of the question to decide for translation or to, maybe, not accept the answer and wait for another one.
I think it's safe to assume that people in this group know at least a bit of Italian and English (and it they happen not to know one of the language at all, they would state it when asking a question). But many are not confident in writing in one of the two languages; at this point it's safer to compose a good answer in their own language, rather than a flaky one in the other language.
I do not totally agree. The goal of SE is also to provide a knowledge base which can be further accessed also by other members and guests, which may be interested in knowing the answer (see the famous 'Bravo!' question regarding female performers) even though they don't know that much of the language the answer is done with. If the answer has to be in a different language, i still would like to have short answers at the beginning (like an excerpt) written in the same language of the question.