I don't think that it's a good idea to mix languages on the site, i.e. letting people ask questions in the language of their choice. Also, we don't have enough interested people to be able to have separate StackExchange sites for different languages. What we can do if someone's english is bad is to edit the question to help them make it clearer and more correct.
So, don't instantly downvote a question if the language is really bad but there seems to be a legit question hidden underneath. Instead, edit it! If they don't agree with your interpretation they can re-edit it or let you know in the comments field. Remember, this is a collaboratively edited Q&A site.