Base language of Python

Question

What is the base language Python is written in?

Answer 1

To add to and reframe some of the other good answers:

The specification for Python (question) is written in English, but could be written in a formal semantics, as Standard ML and Scheme are. See Programming language specification.

There are implementations of Python in many languages, as noted by Gandaro, of which the fastest is surprisingly not the original CPython, which is written in C.