Unfortunately, the "system" of XML documentation is not so flexible.
I do not have a good solution for you, but you can take a look at using some XML editors to take the source XML created by Visual Studio and translate it into German - if the XML structure remains the same, you can use (SandCastle or something else ) to create German documentation.
Update:
, @ta.speot.is comment .
, , xml:lang <summary> - XML, XSLT .