BuddyPress.org

For the backend it’s simply a matter of adding another .mo file to the language folder in /wp-content to get wpmu to be multilingual, the same goes for any other plugins, they each have to have their own .mo file.

There is a great plugin available which makes translating plugins or even the the wpmu core itself easy, and allows you to do it a little at a time. CodeStyling Localization (I am not affiliated with them in the least, but it is more than great). It allows you to do localization for themes, plugins, mu-plugins, and wpmu base code itself, absolutely awesome.

http://www.code-styling.de/english/development/wordpress-plugin-codestyling-localization-en

For the front end and to allow different users of any of the blogs to change the backend easily and quickly I recommend qTranslate.

http://www.qianqin.de/qtranslate/

It is easy to use and easy to setup. Non-technical users will immediately know how to use this once it is setup. The setup may not be for non-technical users on their own, but a simple how-to will easily walk anyone through it. This plugin allows visitors to see multilingual content and change the language on the blog front-end (although I haven’t really used it for multilingual switching for the social networking portion of buddypress, I’ll have to play around and see what it can do).

Each blog can have it’s main language set in the the Settings/General page with the site wide default set in Site Admin/ Options (this is built in to wpmu, it doesn’t need a plugin – it’s been there from before 1.0 if my memory is correct).

I use English and Japanese (which will often not work when the localization is not done right) so I think these are pretty good plugins that work well with wpmu’s built in localization.