While this will probably not get implemented due to moderation difficulties and slow implementation of engine changes(much longer than 6 to 8 weeks), I'm leaving this for reference and CW'ing it so future users can improve this idea if the functionality to fix it arises.
Disclaimer: I've had to learn English as a second language, so my opinion here may be influenced by personal experiences.
I see another alternative:
While posts will be in foreign languages, Stack Exchange could partner with a machine translator like translate.google.com and provide translations for untranslated posts. These, of course, are not ideal and should be replaced with hand-translations as soon as possible. If a bug can be filed to get error text to be given in English, and the user's language, at the same time, that would be very helpful. Strings, quotes, and parts of the post body even can be searched fr a translation through Launchpad translation data, culled and imported every month or so.
Translation experts, avid editors, and native speakers can translate posts into English, and possibly translate answers for the OP as needed.
Of course, we won't be able to translate every single post into every language for future users, but I feel as if the OP's language and English are enough.
Flags can have a policy to only allow flags in English, except those without text related as part of the flag. I feel as if looking at the length of a post and punctuation/linking(question marks vs short posts vs things that look like "me too" or "thanks".
We can also try to implement review queues for foreign flags. If a flag arises in a language which no moderator speaks, then if the mod deems it appropriate for a 2K or so review queue, then it will be sent, filtered by languages.
We can also try to expand search by trying to use some sort of machine translation to search across languages. Maybe internal machine-translated English versions can be held in the database, and a foreign search will cause the query itself to be machine-translated and compared to the internal DB text. Query expansion, if used sparingly would also be a good idea.
Scores (or user prefs) should be kept to see which users speak what languages, and how effectively. This can be done based on post-voting just as a user preference.
Tags would need to get a new feature implemented: easier tag synonyms across languages.
Currently, tags take a while to get synonymed. Tags synonyms across languages should be put in a review queue, and require not a score in the tag, but a score in that language (or understanding of the language as set by user prefs) should be required to make a synonym.
I'd really love to see feedback on this, so vote or comment, whether voting up or down. I don't mind a downvote if I can hear personal opinion.
A second alternative:
We can keep one site, but use Area51 for a slightly different purpose. Instead of creating new sites, it will allow that language to be used on the site after passing a review and moderation success "beta", given enough interest in the language.
Luis Alvardo brings up a good point:
@ObsessiveSSOℲ - There is an additional problem. If the user writes the questions with a very bad grammar (Like it happens a lot in english or spanish for me), the translator might go crazy and translate that into something even worse.
The machine translation will only stick around until hand-fixed, either by tweaking the translation, or hand-translating. If the translation becomes offensive or totally incorrect, it can be brought up by sending to yet another review queue for translations. That's the trouble of a multilingual site.
In the end, I feel as if this will help make Ubuntu more usable by all. We'll also start to see more moderation responsibility by mid-to-high-mod users, and a focus on multilingual elected moderators.