A German court can not, in fact, serve a court order to Volkswagen America. (Technically it can, but the order is not enforceable.) A German court is also forbidden to force Volkswagen AG to reach into Volkswagen America because these are separate legal entities.
A German court would use judicial assistance and ask US law enforcement to help. US law enforcement would then adhere to local laws and protect the rights of US citizens while trying to help.
A German court would use judicial assistance and ask US law enforcement to help. US law enforcement would then adhere to local laws and protect the rights of US citizens while trying to help.
This is named Mutual legal assistance: https://en.wikipedia.org/wiki/Mutual_legal_assistance_treaty .