C++! The browser you are using is probably written in C++. Or it's compiler. Don't forget about your operating system, which probably uses at least some C++ (and some C or Objective-C)
What about embedded? You will have to learn C, or no complex embedded software for you. So C is a must-learn, too!
Since todays industry is all over Java or Python, if you want a job in certain industries, you must know them. They are must-learn!
PHP! Want a small site with only very simple and small generated piece of content? Anything but PHP would be an overkill! So it is a must-learn!
I could go on for a number of other languages. No language is a must-learn in a general sense, so please explain your circumstances :)