In my experience, it's not just one language. A lot of products are built in a variety of languages, based on what the need is. Python for testing, Haskell for

In my experience, it's not just one language. A lot of products are built in a variety of languages, based on what the need is. Python for testing, Haskell for static testing, C++ for high-performance low-level stuff, Lex & Yacc for, well, lexing, etc.