BuddyPress.org

from the WP org codex:

Using Localizations

In order to localize your installation of wordpress, create a directory named languages inside of wp-includes, if it does not already exist. Then grab the appropriate localization files from the Subversion Repository as described above. The main .mo file for the language should go inside the languages directory. Set WPLANG inside of wp-config.php to your chosen language. For example, if you wanted to use french, you would have:

define (‘WPLANG’, ‘fr_FR’);

i’ve this configuration on my MU+BP and all works fine.