I have come across a few questions that were asked by non-native English speaking developers who have asked questions that can be interpreted in multiple ways. Basically the questions asked is an answer for why something 'is this way' and not why 'does my code not work'. I feel these developers are trying to reach the collective knowledge of stackoverflow for an answer they cannot articulate. In an ideal world all questions would be translated in all languages so the best person could provide the most expert answer. But with localization stackoverflow would segment awesome questions and answers based on where you live depriving all users of stackoverflow.
It's a small leap to take Metcalfe's law from devices to people; sites in the SOFU trilogy are awesome in large part because someone is an expert on every question. (No, not Jon Skeet.) Area51 recognizes that the size of a social network is immensely important in forming a self-sustaining community, and thus requires enough users to commit to the site as a way to gauge if it can become a viable community.
English is, for better or worse, the Lingua Franca of programming for the time being, and asking programming questions in English is going to expose the question to the largest possible audience.
If the sites are fragmented in SO-English, SO-German, SO-Russian, SO-French, SO-Indo-Aryan, SO-Dravidian, etc., will any one of them be large enough to claim the experts necessary to become generally useful enough to become the go-to programming site for that language? Maybe. Maybe not. I'm skeptical.
But another part of what makes SO great is the high quality standards we hold. We expect questions to have sufficient details to be answerable. We expect a certain amount of research when asking questions. We expect answers to be more than vague guesses. Answers that explain are held in very high regard. If you take away the community that enforces (or at least encourages) these standards, you might not get these same quality levels in the smaller communities.
It's an unfortunate fact of life that language is sometimes a barrier to communication. The best we can reasonably do is to: