Rotmg Language?

What language is rotmg coded in. I am guessing multiple languages like actionscript, c#, and c++. But What language are the things commands coded in. I thought it was java or c# or c++. But if I want to learn how to code rotmg advanced, what language do I need to learn?

For sources out there C#, as Bledi stated. Personally I think that simply looking through the source will help you understand what's is what, certainly is faster then reading a massive book, and more fun and motivating as you get to try out stuff along the way.