Which language should learn to work on functon node programing

I want to know that which language should learn to work on function node programing, l

Node-red runs on nodejs, which uses Javascript. So you would use Javascript in the function node. You can reference this site for functions/methods/etc

Also see Writing Functions : Node-RED

I recommend watching this playlist: Node-RED Essentials. The videos are done by the developers of node-red. They're nice & short and to the point. You will understand a whole lot more in about 1 hour. A small investment for a lot of gain.