radio license for the pilot
One can reasonably assume that all N-regs in Europe have a radio station license on board. You find this out when you do your very first AR
ROW check on it.
The RT license for an N-reg pilot is also issued by the Federal Communications Commission. It can be obtained via the web with a credit card. It costs you about $60 now and doesn't expire anymore (the old ones needed a 10-year renewal).