My thoughts on this are that the majority of this sites users will speak English, one way or another. So if you want to get the maximum amount of answers to your question, then English is probably the way to go.

When answering, if you know the asker speaks another language, then you could use that, however your then going to prevent others who don't speak that language from using your answer.

So I wouldn't say no, you should never use anything other than English, but to maximise the effectiveness of your question and answers, English is probably the best way to go.