How to run user code on server in different languages?

Question

I would like to make an application simmilar to CodeFights in that a user is given a set of excercises for which he has to code solutions. I figured out how to take a users JavaScript code and run it on a Node.js server using for example the Function constructor:

try {
   var solver = new Function('arg1', 'arg2', bodyAsString);
} catch(e) {
   console.log('Function cannot be parsed');
}

Good thing about this approach is that the solver(...,...) function now has access to global scope only and not the scope it was made in. Even better one could also combine this with a sandbox module.

But what is the best approach if I want to use multiple languages - let's say JavaScript, Python and C++. How do I go about solving this problem if I want to give the user a choice of language? Do I write his solution to a file and try to execute it via command line also in some kind of a sandbox mode? Can this even be done if I use Node.js as backend?