At area51 are multiple proposals for sites that already exists, only in another language. Actually I've committed for the German unit of Stackoverflow. And that just because I want to provide some of the already collected knowledge to those that cannot speak English.
I'm aware that the automatic translation today will not gain the quality we're all striving for. But I think the sites would gain more quality if the users are able to translate every single post into English or their native language.
So I will explain my vision: The main language will be English, but everyone can post in native. The posts will drive a new queue where reviewers just have to decide if there is a translation into English needed. Answers in English have to pass a queue which only native speakers can review and translate it from English. If a post is available in another language than English, there will be an option to show it.
No matter how many non-English speakers would post questions in their native language, "normal" users will not be disturbed by post they cannot read. Nonetheless more users could participate, so it would help to grow the community and improve the quality, because forcing someone to use a specific language will reduce the amount of good questions. Further the time to get an answer in cause of the queue-passing will encourage the users to post in English. Rewarding users for approved translations will boost the range of people which could gain knowledge.
After posting this, I've found the results in the related link list which I've searched for before, so myself flagged this one as duplicate...