In Polish, you don't use the plural vy as a formal singular. In Russian you do. I took Polish lessons at one point and my teacher would wince every time I accidentally called him vy. He told me the Russians used to force Poles to address each other that way in their own parliament. So sensitive was he to this long-past linguistic oppression that he couldn't help but be offended when an English speaker who happened to have studied Russian did it by mistake.