Has the European ROM been dumped yet? I know there's German and French ROMs already, but since I can't read those...
EDIT: I uploaded my save from my retail European White 2 and tested it with an American White 2 ROM and it worked just fine. I'm going to assume that an American save will work with a European cartridge.
If I were to start playing Black 2 on my flashcard using a US ROM, and I later buy a EU copy of it, could I transfer my .sav to the retail copy and keep playing? Or do American saves only work on American games?