You don't need an external unit. Window AC (including casement sized) exists as well as portable AC.

Most European windows open sideways. Scientists are stumped with not solution.

That doesn't matter for portable AC. Most people build a plexiglass or foamboard plug for the open window. Some products are sold already set up like this. If you can fit a casement AC that also probably doesn't matter. You can probably remove the window too I'm guessing.

Portable AC exists and people do use it, but it's loud and awkward with casement windows. This is a bandaid solution at best. Yet, I've seen them sold out in the summer, so people do use them.

I actually have a casement AC unit as well.