What is the programing language I'm editing in when make an HTML5 Canvas file in Animate?

The entire point of HTML5 Canvas documents, unlike SWFs, is that they don't rely on a browser plugin. That means they can only do what browsers themselves can natively do. So that means they use the only language browsers can use: JavaScript.

https://helpx.adobe.com/animate/kb/as-to-html5.html