Hi NikaBee,
here you find the constitution of JW Germany:
http://www.jehovaszeugen.de/uploads/media/Statut_.pdf
According to Statut §15(1), you left "JWs in Germany" already when you went abroad and associated with a JW' congregation in the US! Of course, from WTS point of view, you were still a member of JWs, but you were not a member of the legal entity "JWs in Germany" anymore. So it is not necessary for you to contact any state authority.
Just some additional background info, if anyone is interested in:
Germany (as well as some other European states) wants to ensure that anyone can end church membership at any time. But some churches (e.g. Roman Catholic Church!) do not allow termination of membership according to their church laws. Therefore, the state names a worldly authority ("Standesamt") to be responsible to receive someones declaration of leaving a church. And the state obligates the churches to accept such termination of membership (e.g., Roman Catholic Church has no right to receive church tax from anyone for the time after he left the church at Standesamt).
But, contrary to Roman Catholic Church, "JWs in Germany" (as JWs all over the world) do accept that any member can end membership himself
- by sending an accordant written declaration to local body of elders, or to Bethel (Statut §15(2)),
- or even by just verbally declaring to leave JWs, if there are at least two witnesses for such a declaration (Statut §15(4)).
These are the common ways for disassociating from JWs.
Additionally, according to state law, there exists the possibility to disassociate from JWs be going to state authority ("Standesamt"). But this is very rarely used among JWs, as it is more complicated than the possibilities for disassociation provided by JWs. At Standesamt, you even have to pay some fees (I think, about 40 USD) to state authority. But, of course, if any JW who wants to quit doesn't want to send a disassociation letter to local body of elders or to Bethel at all (as he feels that sending such a letter to elders would be equal to accept authority of elders?), he might prefer to go to "Standesamt" and to declare there that he disassociates from JWs. "Standesamt" would inform Bethel from such a declaration, Bethel would inform the local congregation, and the result is the same (disassociation; announcement in congregation as usual: "... is no longer one of JWs")