BuddyPress.org

You need to have the WordPress language files in wp-contents/languages first, before WordPress will give you the option of changing the site and blog languages to something other than default. Once you have the files there, set your Site Language in Site-Admin->Options, then set your blog language in Settings->General.

Basically BuddyPress will follow whatever WordPress MU is set to do, but it cannot tell it what to do (yet).

WordPressMU has a specific way of handling language settings that involves a function that overrides the typical get_locale().