NEWBIE - Move Data fields in input form
there is a section in the Developer Guide on templates: https://docs.civicrm.org/dev/en/latest/framework/templates/

This includes some pointers on how to customize templates.
Having said so, you might also be tempted to put this in an extension as you might run into some more "Germanization":-) Are you aware of the German CiviCRM mailing list?