F# - Functional language where you can cheat more than haskell if you need to. Can use all the .net ecosystem and works everywhere (linux, windows, mobile, can compile to js.)

C - Pretty much everything is built on top of C right now.

Javascript/html/css (using Flow to add a robust type system, since dynamic typed languages are shit for long term ease of development)- Required if you want to do anything web plus its the only true cross platform GUI to use on top of your F#.