How to provide German translation for menu items and legend

[D. Schambach]

Hello,

I am German and have two kids about to leave elementary school (hence 
their English is not that good, yet).

What would I have to do to improve the translation of the menu 
items/dialogs and the legend into German?

Also I would like to know, how to make names of mountain ranges (e.g. 
Black Forrest/Schwarzwald) and names of rivers (Rhine/Rhein) appear on 
the map. Is there a plugin or would I have to provide the information 
myself? In case of the latter, how would I do this?

I studied the user manual and part of the other documentation, but 
haven't found the answers I was looking for.

Hoping someone can answer them.

Kind regards,

Dirk