Bitcoin Core was originally written in C++.
Satoshi Nakamoto’s original Bitcoin software was released in 2009 as an open-source program. C++ provided the low-level control and performance needed for a peer-to-peer network that verifies transactions, stores blockchain data, and performs proof-of-work calculations.
Bitcoin Core remains the leading reference implementation of the Bitcoin protocol, although the project has evolved substantially since its first release. Modern versions include code written in C++ along with supporting tools and scripts in other languages.
People sometimes confuse Bitcoin Core with Bitcoin itself. Bitcoin is a protocol and network, while Bitcoin Core is software that implements that protocol. Other compatible implementations can connect to the same network, but Bitcoin Core is the best-known one and is maintained by a global group of contributors.