In french there are two ways to say you. You use “vous” when you talk to sb in a formal way and “Tu” if you talk to sb in an informal way. In German you use “Sie” as an formal way to speak to sb and “Du” as an informal way. The formal you is used when the person is older than you and/or if it’s a person you have to respect. You would use the formal you to talk to a teacher for example or for people you’ve just met. Sometimes one person offers the other person to say the informal you to each other. So, the informal you is used among people who know each other well, as for example friends and family members.
