First, you should use the language pack from www.osTicket.com/download and second let me mention that you need to set up a fresh installation with German as the primary / install language that the email templates and so on show up in German.
At the moment it is not yet possible to use the German email templates etc. on an already existing English osTicket installation by adding the de.phar. This will for sure change with some of the future versions, but for now it is not possible and you need to set up a fresh installation.