Actors assume accents that aren't their natural language/heritage frequently in movies and on TV. Obviously, some of them are better than others. Some are downright laughable, but some are extraordinarily good. It seems to me that even an almost perfect imitation of, say a German accent, would none-the-less be detectable as fake to a native German-speaking person.
However, when I first started watching "House", I was so confused because I had thought Hugh Laurie was British. I became convinced that he was really American and had faked the British accent in the past. He was THAT good.
So the question is:. Could a talented actor use a foreign accent so well that someone who spoke that language his entire life couldn't discern a difference?