See: Is it hopeless to start a non-English site?
If you're talking about a fully localized size (such as, Stack Overflow in Hungarian), we don't have support for that just yet at all. We're working on some localization projects (most recently, support for non-ASCII tags), but we're still a ways out from being able to support a fully non-English site.
Short answer is that there's a lot of dev work needed to support localization before we can even start on the task of actually localizing (which is going to be even more work) - we're working on tools now that'll allow bilingual folks to help in constructing a site for those who can't really use English effectively at all. This isn't a quick or easy task, but obviously there's considerable demand for it so we're pushing forward as quickly as possible. Remember, we're retrofitting a system that was designed and built only to support English - this has some significant challenges (remember all those pluralization bugs folks reported?) that we'll probably go into detail on in a future blog post.