The examples I'm surprised no one has mentioned so far: C and C++. (gcc is written in C. clang is written in C++. Just about every C or C++ compiler is written

The examples I'm surprised no one has mentioned so far: C and C++. (gcc is written in C. clang is written in C++. Just about every C or C++ compiler is written in C or C++ or both, though there are exceptions like the C compiler on the Symbolics Lisp machines.)