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technically none... You can use Linux without knowing how to program. Just learn your Linux distro on their official site or some walkthrough and you'll be good to go! Learning terminal commands and shell scripting may help a lot; thats the closest to a "programming language" you'll need.
Question is too broad. What aspect of linux are you trying to understand? Check this out: https://askubuntu.com/questions/324250/what-computer-languages-do-linux-based-oses-use
Understand how to use it or understand Linux kernel programming?
Actually there is a difference between GUI and CLI. GUI stands for Graphical User Interface. (Windows, MACOS) Where, CLI stands for Command Line Interface. Linux is CLI based operating system. In that, you have to type commands to proceed your task. So there is no programming language required for linux. You will learn commands of linux from this link (THIS MIGHT HELP YOU :- https://www.google.com/url?sa=t&source=web&rct=j&url=http://cc.iiti.ac.in/docs/linuxcommands.pdf&ved=2ahUKEwiOpbuN68HfAhVGXCsKHbaNBKcQFjAAegQIARAB&usg=AOvVaw1z73BU1N6nsX-L5pyniuaW )