To renew registration, you have to have a California address and you have to have insurance in California--which doesn't do you any good in Europe. While I have a U.S. military registration and thus international insurance for my bike (linked to the VIN number, not the license plate), I also dish out $ 78 a year for stateside insurance (Geico) in order to keep my personalized CA plate although that insurance isn't any good overseas.
By the way: If someone imports a vehicle from the States to Germany, they can drive with stateside plates for up to one year after clearing customs and purchasing international insurance (green insurance card).
I have always been able to keep the old plates when buying a used vehicle in California--but it has been a long time that I bought a used one there, so that might have changed.
