Steve, if I understand your question, you’re asking how one could go about enforcing a judgment against Germany, assuming that the plaintiff ultimately gets a judgment. I’m assuming that this is an academic question and that you’re not looking for legal advice about any particular case.

In general, US judgments for money damages are enforced in the United States by writ of execution, but when the judgment debtor is a foreign sovereign, the FSIA makes foreign sovereign property generally immune from execution, with exceptions. You may want to look at 28 U.S.C. §§ 1609-1611 to get a basic sense of the rule. Of course, one could also seek recognition and enforcement of the US judgment in a foreign court, subject to the foreign country’s laws, but I’m not sure that that was your question.