Answer 1

Since it runs on Debian-like OS, you can practically use any language you want. It is True though, that Python has by far the largest collection of libraries which makes this process really easy.
I for once use Python on my RPi and taking the performance aside, it works really well even for some of my data science'y stuff.

Answer 2

you can use any computer that meets the specs required by your project.
Since it is your project you should already know if you can or cannot use the rpi.
Btw, in industrial systems i often saw PIC and AVR microcontrollers so i guess is safe to say that you can do some engineering project around an ATMega328 (mcu on the arduino uno)

Answer 3

Depends on the project. The processor is quite alright and ARM architecture is by far enough for most applications. However, my data computations lack adequate RAM and tend to be a bit laggy sometimes.
Still, a powerful tool for such a small and cheap setup.

Answer 4

Check it out:
https://www.raspberrypi.org/blog/tag/robots/

Answer 5

What you mean by "high-level" then?

For time-critical microcontroller handling it's perhaps Arduino which might be more adequate for your needs..?