Edited the German localization

The German localization was wrong or didn’t make sense at some points,
or used a less polite expression for “you”. (Du/Sie, Dir/Ihnen,
Dich/Sie, Dein/Ihr, …) 
I'm a native German speaker so I guess I know what I'm talking about for once ;D.

I could create a localization in the less formal form if anyone is interested.