survival Japanese

Although all Japanese people want to speak better English today, we find it difficult and are sometimes at a loss when talking with foreign customers. For this reason, it can come as relief to discover that a foreign person speaks Japanese.
I have a novice student, whose family name is coincidentally the same as common Japanese one, like "Suzuki" or "Tanaka," though her ancestry has nothing to do with Japan. She says that when she meets Japanese people for the first time, they look relieved to know her surname, happy to assume that she speaks Japanese, and begin to speak in it mercilessly. The first thing I taught her was, "I don't understand Japanese," in Japanese.