An unusually skilled mimic, certainly espionage agent, could easily fool even the supposed experts into thinking that they're native to the language as well as the culture to which they are pretending to belong.
How about that woman, Chapman, here in the State Dept., who successfully fooled both the FBI and the CIA until she tripped herself up (or gave herself up, can't remember which)! Turned out she was a Russian agent here in the States:-)
I can't think of any foreigners who speak Polish that may be taken for native speakers. I may be wrong, though. Ukrainians (and other Slavs) seem to be the closest, yet they often have this zaśpiew that reveals their foreign origin.
