Where can I find out what language a MCU would use?
Microcontrollers don't "use" a language.  They execute machine instructions that are stored in binary in their program memory.

If you want to code in C, then you have to use a program that converts the C you write into the machine instructions the micro will execute.  This is called a compiler.  You probably also need a linker to go with that, although compilers and linkers usually come bundled together.

However, you need to learn how computing machines really work first.  On a big machine with a operating system, it's useful to understand what a compiler, librarian, and linker do, and what the machine itself ultimately does.  On a small machine like a microcontroller, it's essential.  Without this understanding you're not going to accomplish anything with a micro.