What is the language of Arduino IDE?

Answer 1

Alex, you can find more information on this item here:
https://www.arduino.cc/reference/en/

Answer 2

C or C++?

Answer 3

It's kind of a mix of C/C++, along with a few of its own keywords. Really, if you know either you can learn Arduino easily.
(I learned Arduino with a book and am now taking C++ here).
Good luck!