European windows open sideways not up and down. So we can't just stick an ac unit on the window.

Yes you can (I've got one), there are various window seals to adapt to that or you can dyi too. The noise etc make it a temporary solution at best but it's possible.

I've also used portable AC inside my home, it was horrible, a lot of heat creeps in due to bad insulation for the heat exhaust pipe, also creates vacuum since it exhaust air, which in turn sucks in more hot air

You don't stick a portable AC unit in the window. The unit sits inside. You just run vent hoses out the window for the compressor.

Opening sideways also shouldn't matter for a casement AC unit.

Portable ACs are borderline scams because the hot side is inside the room it's trying to cool, among other reasons[0].

[0] obligatory TC rant: https://youtu.be/_-mBeYC2KGc

That's a perfectly good way to waste kW of energy every day and make the heat outside worse.

It may waste energy, but if so, it's wasting y kwh of energy per day (or joules), at a probably-variably rate of x kw, where y is x * 24 (for kwh)

Sure beats heat stroke and death.

You are just moving heat strokes elsewhere.

Hell, people are dying now in Europe in part because US AC abuse burned so much energy it contributed in a large part to global warming.

I doubt AC "abuse" is relevant compared to just about everything else contributing to global warming.

You'd have to have your windows open at all times with only a thin fabric blocking the opening to do that with most european windows. And then you still have your compressor inside generating heat.

You can shore it up however you like. Some people use plywood or foam board or plexiglass.