Czech (and Slovak, mostly also Polish) use formal/informal you similar to German or French. (At least from my limited understanding of those.)

Formal: High schools, universities, work environment, courts, etc. You also use it when you're speaking with older people or when you want to show respect to person you're talking to.

Informal: Everywhere else. It is also used when you want to indirectly insult person where formal should be used.

Life hack: You can use informal absolutely everywhere when you're old (even when it'd be very disrespectful otherwise) and nobody gives a shit.