Can you point out where is the requirement or discussion that questions and answers should be in English? I didn't find it in the FAQ (except "Languages other than English (including translation)") or About.
There's also a blog entry about that here:
Because English is the language that most users speak.
I’m slightly confused by some other comments/answers. There seem to be two different questions that are getting conflated:
What can one talk about on a site?
What languages can one use on a site?
These aren’t necessarily the same! This site is about English — but one could also have a site for discussing Greek in English, or for discussing English in Russian, or… And most of the Stack Exchange sites aren’t even about a language at all, of course.
However, it’s also the case that the working language of this site is English. Why? Jeff has argued elsewhere for why they are trying to keep the Stack Exchange sites monolingual for the moment. And English is the language that most people in this community speak best — so English is the language we choose.