Not a joke, cygne, a true story, reported by the woman herself on NPR:

She was a gaijin in Tokyo asking directions, in fluent Japanese, of a Tokyo policeman. The policeman responded, also in Japanese, that he was sorry but he couldn't help her, as he spoke no English. She responded, "Yes, I understand that you speak no English, but, if you listen very closely, you'll notice that we are speaking Japanese." All this while remaining politely correct as a woman speaking to a man of authority.