BuddyPress.org

1) Download your BP language files from:
https://i18n.svn.buddypress.org/

Make sure you grab the release that is specific to your BP version

And add them to /wp-content/bp-languages/

2) Add this to your /wp-content/plugins/bp-custom.php file:
define( 'BPLANG', 'YOURLOCALE' );
if ( file_exists( WP_CONTENT_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' ) ) {
load_textdomain( 'buddypress', WP_CONTENT_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' );
}
—
Just tested it, works for me.