Is it that hard to drill an 8cm hole to run some refrigerant pipes through the wall?
Most of Europe has a "registered building" system, where buildings above a certain age are considered historic. Renovating those buildings is an extremely difficult, expensive, and bureaucratic process. You generally need to preserve the period-appropriate look and materials. An AC unit sticking out of a wall won't pass muster.
Even newer buildings are problematic. an acquaintance of mine lives in an upper-middle-class apartment complex that was finished two or three years ago, and their architect has some claim in the contract that prevents residents from installing AC units to "preserve the building's unique look."
The US is build around privately-owned housing (and hence creature comforts) a lot more than we are, so AC is a lot easier to implement there.
It feels weird to have to mention this, but although there are a lot of historical buildings in Europe, it's not the norm.
Population grows. There was a noteworthy war not that long ago. The vast majority of buildings are not that old.
And just to be clear, there is a huge difference between a person who has a keen interest in history (nothing wrong with that), and one that wants to leave every single old building untouched, and if there's a danger to residents, move the residents.
If you have a balcony, you can mount the external unit there, but then you need to route the piping to the room(s) that need AC. Then you need to cover the pipes somehow, which is still perfectly doable, but goes way way beyond "drilling a hole".
You can have a portable unit, but it's loud, takes up space indoors and needs to be stored somewhere most of the year when it's actually cold rather than super hot.
It’s ~25kg for a 2.5kw/9000 btu condenser I just looked at, but yeah, if there’s no balcony/fire escape, and you’re well off the ground, you might need some rappelling gear, and you might want a pro. No idea about whatever heavy bureaucracy you live under, that’s a human imposed rather than fundamental challenge. Pipe routing isn’t a big deal, the copper lines are very flexible, just need to be careful not to kink them - give the bends a good radius. And to answer your unstated objections, you will need some tools you almost certainly don’t have lying around. Mostly, though, you need an attitude of being able to do stuff, and getting it done.
Or you can get a portable or window unit. Midea makes some really nice window units now. Might need hung windows, though, not sure whether it’s possible to make them work with casement windows. It takes a little space, but that doesn’t seem like a huge deal?
Or do nothing and just deal with it.