Re: Localisation, per user
No, you only need one installation of WPMU/BuddyPress to have a multi-language website.
When you install additional PO’s and MO’s in your wp-includes\languages folder, magically in your Site Admin and Blog Admin settings will be language selection options, to determine the default language of both the site and the blog. The mu_locale function will take control of it and over-ride the WPLANG settings as necessary.
There is no user language settings as of yet, but I’m working on an implementation of qTranslate to allow for this. This won’t work until BuddyPress 1.0.1 as there’s a few specific things that need to be altered for everything to work in the order they need to.