All too often, people speaking their native tongue in a foreign country assume that no one around them has a clue what they're saying. In reality, you never know what knowledge any given person might have inside their head and what languages they might know outside of their homeland's. In addition, restaurants are a very common place to unintentionally overhear others' conversations, and thus catch someone in the act of talking trash in another language. Whether it's an older Japanese pair…