Unfortunately the XML documentation "system" is not that flexible.
I don't have a good solution for you, but you can look at using some XML editors to take the original XML produced by Visual Studio and translate it to German - if the XML structure remains the same, you can use the normal tooling (SandCastle or whatever) to produce the German documentation.
Update:
The approach that @ta.speot.is linked to in his comment is interesting and can be made to work.
In general, you have a complete copy of the documentation comments in a different language, only using an xml:lang
attribute on the <summary>
element - the produced document will contain all the XML elements and you can use XSLT to transform and select the language.