BuddyPress.org

You should be using the new BP_PLUGIN_DIR constant, which was introduced in revision 1303 for moving to MU 2.7.1 support.

Something like:

define( 'BPLANG', 'spots' );

if ( file_exists( BP_PLUGIN_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' ) ) {

load_textdomain( 'buddypress', BP_PLUGIN_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' );

}