I think the only way you could pull this off is if the application in question could be constrained in a window that your own application creates. Since X-Plane is a game, and since most game companies would never even consider running their game in a window that wasn't borderless and full screen, this is going to be quite the feat of magic. However, I've always been fond of saying that if you can think of it, it can be done in software.
As far as language, OS-X is nothing more than a proprietary version of BSD, but being BSD, it opens you up to be able to use any language available for the base OS. I would probably go with C++ since it's widely used and support is good, not to mention the existence of at least one free compiler. I also found this link:
The paragraph that talks about re-routing system calls is probably going to be important. It provides a couple of links (that I did not follow).
If you manage to get this thing flying (no pun intended), be sure to come back here and post an article about how you made it happen.