If anyone else was confused and was too embarrassed to say it out loud, I was stubbornly typing exit() to try the other languages before I actually read the menu. You have to click on the different REPL names to switch between them. And this link automatically starts you in python3.

This is really great though. Are you planning on adding any other languages? or alternative REPLs? For Python, the option to run bpython or ipython would be fantastic. As far as other languages, I could see this as being a savior when I have to fix something written in PERL or PHP once every 3-5 years, and I need to relearn the syntax. Being able to choose arbitrary older versions of languages would be good for that use case too.

I was going to ask more questions, but then I looked around the site a bit. Is it fair to say this is essentially a demo for the CheerpX emulator? Which started as a proprietary way to run legacy flash applications on modern browsers? If so, then my next question is if there is any chance of some or part of this becoming open source?