Answer 1

Languages can be written in themselves (PyPy, for example, is Python written in Python), but they are commonly written in either another language, or another language and the language being written (CPython, for example, is Python written in C, but with bits and pieces (especially in the standard library) written in Python).

Answer 2

You might be able to submit changes to a language, but you'll need to be extremely proficient in both the language you're submitting changes to, and the language that language is written in (or whatever you're using to try and improve the language).

Answer 3

You should also look into how compilers are built. Building a compiler is a major part of creating a programming language. This is what is sometimes taught in university after learning Computer Theory.

https://en.m.wikipedia.org/wiki/Compiler

https://en.m.wikipedia.org/wiki/Theoretical_computer_science