Germany - Deutschland, but in English

Depends entirely on where exactly you are. The only place you can be certain you're allowed to camp is official camping grounds (Campingplätze / Zeltplätze). Everywhere else MAY tolerate you for a night (unless you behave like idiots, of course) or they might chase you off or even, if you're unlucky, fine you.

(Disclaimer: not legal advice in any way, just based on general experience from living here and ViBeS) I would assume that sleeping in your car would be the safest route if you're determined to avoid fees. People are less likely to assume you're setting up permanently but will be more willing to believe that you're just passing through and couldn't find a place for the night or whatever.

a friend told me about 1nite tent the other day. It's a website where people offer their private land (like gardens) as a camping site for free. Seems to be quite popular in germany.

Sleeping as part of a car journey in the car is allowed. Wild camping is generally forbidden although Bivak/Sleeping bag in a Schutzhütte perhaps even in a bus stop is allowed as part of travel.

All that said if nobody notices you, there's gonna be no trouble, and even if somebody notices your camp the worst they'll likely do is tell you to pack and leave.

Otherwise try sleeping one driver on the ferry i guess.

I have done wild camping with friends on bike trips before. It's technically not allowed but most people don't care as long as you're not in a public place. We'd always pick our spots in the tree line between fields and set up our tent there. That was pretty far off in the countryside though. Be aware though that wild boars are very common in Germany and can be dangerous. Being woken by one screeching right next to your tent can be quite the experience. I believe that legally, the worst thing that can happen if you're caught will be a fine. Most likely you'll just be asked to leave though.

As others said Its technically illegal to camp in the wild. However, if you dont get noticed nobody will care and if you dont behave like assholes and leave no trash behind nobody will care too. However, I do suggest, that you search for a spot that is not farm land, since most farmers absolutely do not like it when foreigners camp on their land without asking. And it might cause trouble if you get caught.

It might be difficult to find a place where you can be undisturbed, especially with 3 people and a car. Maybe try Park4night and stay on a farm. Otherwise you would have to find a good place to hide to not have upstanding citizens freak out and call the cops on you, or find the corners where the homeless people go and that might not be the experience you are looking for.