There is no InSim programming language. InSim itself is a protocol, which is basically just a way of doing things. InSim programs can be written in any language which is capable of creating a network connection, which is pretty much every language worth mentioning. LFS doesn't care which language you use to connect to it with, so long as you follow the correct rules when communicating with the game.
There are pre-written InSim libraries around which can help massively with the task of writing an InSim program, so it would be a good idea learn a language which targets one of those libraries. The most prevalent InSim libraries these days seem to be LFS_External (.NET), LFSLib (.NET), JInSim (Java), CInSim (C/C++), and pyinsim (Python). You can find these in the Libs and Tools forum.
So your basic choice of language boils down to:
- .NET (which includes many languages, such as C#, VB.NET, Visual C++, IronPython, and IronRuby)
- Java
- C/C++
- Python
You can search on Google to find introductory tutorials on these programming languages, which should help you figure out which one to learn. I would suggest Python as a good language for beginners, but as the author of pyinsim I would be accused of bias.