To me a non-living thing can't sit at all. The English sentence sounds so weird. I would rather use "is on top of the window" or "is hanging above the window" . And I guess for this sentence you should know that in Israel ACs are mostly the ones that are hanging on the wall under the ceiling and not the ones standing on the floor
In America, there are many air conditioners in windows. That is very common. In my living room, the air conditioner is in the wall (meaning that it goes through the wall). I’ve seen some air conditioners on the wall (meaning that they don’t go through the wall, but are attached to it, usually up near the ceiling. But an air conditioner absolutely cannot be on a window. How would you attach it to the glass? The air conditioner must be in the window. Prepositions are very particular to each language.
As a native English speaker I can say that to sit "on the window", implies that the window is flat on the ground with the air conditioner on top. This is NOT correct, though I am glad to see that DL accepts "in the window" as an alternative, but this should be the only correct answer