Compartments have to be booked (at a separate charge from the actual train ticket).
You can book a compartment as a 1 bed (the most expensive), or a 2 or 3 bed (cheaper). In the case of a 2 or 3 bed booking, you share with another person of the same sex.
Couchettes are a bit different. They are 6 berth compartments (mixed genders) and you don't undress to sleep!
Typically, you have to have a 1st Class ticket to book a sleeper, and a 2nd Class ticket to book a bed in a couchette.
So, it's not the open slather you think. And, there is a conductor per carriage and although he may not speak much English he's normally pretty good at keeping non booked people out of the sleeping car (& even selling you a luke warm beer if you run out)

Dave