The GPX Driving Directions web service (and hence Maemo Mapper itself, which uses the web service) utilizes Google Maps and thus should work with any address that works with Google Maps. To be more specific, the web service uses maps.google.com - it may not work with maps.google.de or any other locale-specific version of Google Maps.

When the driving directions parsing code is moved from the web service into Maemo Mapper itself, you will be able to provide your own URL, meaning that you can use the .de version (or any other version) if you wish (at the risk of violating Google's copyright?).