Found a way myself.
Instead of writing a vbscript you can add two components. The first components adds the english registry keys and the second one the german ones. In order to use only the correct component during install you can set the condition of the german component to "ProductLanguage=1031" and the englisch component condition to "ProductLanguage=1033".
Thats all :)
Of course it is arguable whether the use of multiple components or a script which changes properties used by only one component makes more sense but by using multiple components you can create your registry keys through the designer. Using a script which changes properties will certainly lead to more errors since the script has to work and every property has to be set correctly. In my eyes its more simple by using the components way.