I'm taking a look at this now to see if I can trace a solution. There have always been problems in the way the language packs pass settings of customized numbers back to scripts.
I've an idea on how we can permanently resolve the issue in v2 which will have to wait until then as it will require some of the additions/changes we plan on implementing. In the mean time, if you can, test it by clearing the language settings for your German pack. If that passes the variable correctly, it is the language pack that is definitely causing the issue. If not, then it's something else.
I've got a Spanish pack installed I used for testing the Classifieds money settings on which I'll have a play around with now. How I fixed it for Classifieds requires the changes I mentioned above for vBDonate.
Matt