What you are actually doing is creating a German directory to mimic the English Directory and english.php.

catalog/includes/language/english/

catalog/includes/language/english.php

Example:

define('TEXT_GV_NAME','Gift Certificate');
define('TEXT_GV_NAMES','Gift Certificates');


Change Gift Certificate to Geschenk-Bescheinigung

Do this for any English or German that was not changed in the language pack, or on other pages.