Change the "Send"-Button?

Hi, untested, but this code should work. You can add it to functions.php or add it to an own plugin.

function my_gwolle_gb_write( $output ) {
        $old = 'Absenden';
        $new = 'Nachricht senden';
        $html = str_replace( $old, $new, $output );

        //var_dump($html);
        return $output;
}
add_filter( 'gwolle_gb_write', 'my_gwolle_gb_write' );