Thanks. Yes, other (server-side) languages are definitely coming. We're currently working with a prototype that integrates with Jupyter/IPython, and we're also playing with the idea of using Docker images for giving a full terminal. The biggest challenge is to give a powerful execution environment and make it easy for the viewer to play with the code and be able to run it.
Have you guys seen https://runkit.com/ ? I suspect they are using something like docker for snapshot and replaying. I'm sure there are some tricks to be learned from them