Laura You open or slide it first, then insert and fill the rest or a windows with something.
I find it on Amazon but its not deliverable and has no price. If I googled it it shows air conditions for 350 EUR and more.
You can't slide a German window and if you open it you have a whole of 1 square meter or more in your wall (3x4 ft). I think German windows are different than in the us. Or do you mean I should destroy the glas? We have these very energy efficient windows here with many layers of glass.