Joel asked this question on this blog post. I've looked through the questions in this area here, and seen the official policy, but I don't see anything related to that blog post, so this is asking that question (and I'm going to give my suggestion below).
I think the way to do this is to change the stackexchange software so that you can have multiple languages on one site, not a site per language (per subject).
It would look like this: If you wanted to translate a question, then you could do so. Then, like wikipedia, you could see "this question is also available in these languages". The aim would be that the most upvoted, or community wiki questions would be translated. There would need to be a separate voting system so that speakers of the second language can rate the quality of translation. Similarly with answers.
You could select the language you wanted to see questions/answers in. If there is a question or answer in that language then you would see that, otherwise you'd see it in English (if available). (At a future date we could let you configure a second preferred non-English language.) If you don't configure yourself as a speaker of another language you would probably see no change at all in the site.
If you ask a question in a non-English language then it could be answered in English. If you have English configured as your main language and another as a secondary language, and the question was asked in that secondary language, then you'd see it. Maybe you'd even translate it into English if you're kind (and want reputation points). Of course it could always be answered in the original language.
Although this is a lot of work to implement it gets around the fragmentation of sites and helps those who have some knowledge of English to take a great part in the sites.
(Update: I'm not the first person to have thought of this: Wikipedia Style Localization)
As Pekka says, please take the time to read existing questions and discussions on this. There's been quite a lot of discussion over on Area51:
And here on meta
If local versions of Stack Overflow are to thrive, they must have all-original content. They cannot survive as merely translations of English Stack Overflow.
Stack Overflow is a community and depends extensively on the community nature of the endeavor to succeed. By simply translating some questions to another language, speakers of that language will forever be second-class citizens on their own community, forced to watch the site through the muddy lens of translators, never actually participating in conversations. As an English speaker, I can't imagine being a part of a community in, say, Hindi, where some (probably small) percentage of the content has been (probably poorly) translated to English. I would never feel like I belonged and I would lose interest right away.
There are enough developers who are very strong in languages like Japanese, Korean, and Chinese to create independent, separate sites that cover common problems. And by letting them have their own sites they can develop their own communities.