This shouldn't be that bad, actually--the CoffeeScript Redux compiler should already be parsing things into the Mozilla abstract syntax tree format, at which point we can use the rest of the existing Aether tooling to just run it with the rest of our World simulation.
http://michaelficarra.github.io/CoffeeScriptRedux/ , CoffeeScript.parse() gives the CS AST, good thing is coffeescriptredux is written to be extensible :)
I would be concerned with this feature. Although it is great in theory to support any language, you open up a huge number of support issues when a user is working in python and they run into a problem with a level (for example).
You also reduce the ability for a user to come to CodeCombat and easily help another user.
"I come to Codecombat.com to help another user when they're stuck (Guide class, I believe), and I login and find that this user is writing in Clojure.. which I don't know."
Just my two cents.
@deepak1556 has landed CoffeeScript support in Aether, so now we need #99 in order to let the player specify CoffeeScript mode.
#99 is ready for anyone to add CoffeeScript support to the editor, and we can fix up the LevelSessions and such to know what language the code is in as we see what needs doing trying to hook it up.
Just seen that there is actually an issue for this :) I'm on it just need to fix some small things in Aether to run smoothly :)
I imagine there will be bugs and places (like multiplayer code evaluation) that need updating to know the language, but we can make specific issues for those as we find them.