I know that people often tell others that it is a rule here, but I am have never actually seen it written anywhere, so is it a rule that all questions and answers on this site have to be written in English?

If people who don't speak English though are not allowed to close questions as unclear to them, we shouldn't be allowed to close questions in other languages just because they are unclear for us. If we want to close something, there should be a rule against it, so I think that something about this should be put in the help section, otherwise there is no real justification for closing the questions. And how are users to know that it's an English only site if it is not officially written anywhere?