[21cdb]

Is MultiSite plugin suitable for such purpose? That would be awesome.

I’m also looking for a solution to set up individual buddypress instances with different languages but with the same user base. So a user who has registered on http://www.domain.org would also be able to log in on http://de.domain.org.

Sure the user has to enter his profile details again for each language version he choose, but he can use one username and password to get logged in o whatever language version he wants.

This would be similar on what wikipedia is doing. They have different instances of mediawiki for each language, but you can change between these instances without an additional log in.