Fabian,
You do NOT need to be a US citizen to have a US license!
I know of operators here in England who took the FCC exam, got the American license, and then applied for
(and got) the British reciprocal license!
I don't know if that would work in Germany though!
73's, Stuart, G0SLX / KF6QGL
p.s. I had my British license for some years before
getting the American one! (And I'm a British citizen).
My reason for getting the American license was for my
trips over there!