Let's try solving the original problem first:

What two languages are loaded on the nav?

Have you tried loading 2 x non-German languages to force the overwrite of the German language files? eg GB and FR

What does the rest of the car do when you change the language? ie: what languages display in the check control display?

You problem is strange, as the Mk3 sends the change language command to the instrument cluster, which then in turn sends the new language settings to the entire vehicle. So I struggle to understand where the problem might lie, unless someone deliberately loaded DE and DE onto the nav.