Thank you Eric, that worked. After some head scratching I figured out what you meant with the variable.
This is how my line 362 looks like now:
$WA_redirectURL = (((isset($_POST["shipping_country"]))?$_POST["shipping_country"]:"") == 'Deutschland')?"checkout_success.php":"checkout_success2.php";
I also tried the different languages, and everything works perfectly.
Once more I have to say, you guys from WebAssist do an awsome job.