Plugin scripting is built on top of javascript.
JS is the language to use, with its rules, syntax, standard functions, etc...
Plugins (the SDK) are written in javascript and in such a way that it's interfaced with C2's events system.
This article from Scirra's blog might be a good read too. It clearly identifies each parts of C2 and their interactions/implications.
I can understand the feel to need more than there is actually, but I'm pretty sure text box, button, text, array, ajax, browser and webstorage objects + events system can already let you make nice functionning apps.
What more do you need ?