However, for archiving purposes, it would be good if the posts are retitled in english (though the contents kept in their original language).
Wouldn't this create confusion as to the language used in the post? English-only speakers may view the post expecting english, and non-english speakers might pass it over assuming it's in english. Am I missing something here?
The only conceivable problem I see with native language postings is moderation. What if something that appears to be another language is really just semi-randomly generate strings? Or something offensive (or in violation of copyright laws, etc) is posted and nobody else hear speaks that language? This definately shouldn't discourage native-language postings all together, but it might be best to, at first, stick to ones where there's an obvious level of visitors knowing them (german, french to name two).