function my_custom_bp_mofile( $mofile, $domain ){
    if( 'buddypress' == $domain ){
         $mofile = trailingslashit( WP_LANG_DIR ) . basename( $mofile );
    }
    return $mofile;
}
add_filter( 'load_textdomain_mofile', 'my_custom_bp_mofile', 10, 2 );