Thank you for getting back. It works now :)
I falsely assumed that I needed to implement the redirect on the checkout page. After giving it more thought, it makes no sense, because at that point there is no entry in the database to compare to.
I changed the action on the checkout page to confirm2.php, and the code on confirm2 redirects to confirm.php, if shipping_country is Deutschland, Germany, Allemagne or Alemania.
Thank you again for the great advice.