Finance and esoteric languages

• Jane Street famously uses OCaml, an otherwise-obscure functional programming language.
• Morgan Stanley in the 1970s used APL. (A Demon of Our Own Design by Richard Bookstaber, ch. 3)
• Goldman uses a homegrown language called Slang. (HN thread with discussion)
• Other big investment banks use "Bank Python", their own homegrown forks of the Python ecosystem.

Anyone know of any other examples?

Outside of finance

• WhatsApp is built on Erlang. (src)