Satoshi Nakamoto, the founder of Bitcoin, wrote Bitcoin Core in the C++ programming language. Bitcoin Core is a specific implementation of the Bitcoin protocol (Bitcoin Core GitHub repository), and it's the reference implementation of Bitcoin.
The programming language C++ first appeared in 1985: Bjarne Stroustrup is the founder. It's a general-purpose language: an extension of the C programming language. Over the years, it has evolved as a technology. According to Stack Overflow's 2020 Developer Survey, C++ is the tenth most popular language.
In an interview with Lex Fridman, Bjarne Stroustrup mentioned that he was unhappy about the usage of C++ in Bitcoin. Stroustrup's primary concern was the excessive energy consumption involved in Bitcoin mining.
As Bitcoin is an open-source technology, we can read the Bitcoin source code on GitHub. The most popular languages in the Bitcoin repository are C, C++, and Python. To reiterate, Satoshi Nakamoto wrote the original codebase in C++.