Just a question out of curiosity. What Web framework is used for Duolingo? I place my bet on Django, but I'd really like to know an answer from someone from the team :)
Duolingo uses Pyramid as the python web framework, with SqlAlchemy for the ORM. For the front-end, it uses a combination of mustache templates, backbone.js, jquery and Twitter bootstrap. Here is the answer of one of the Duolingo developers:
(a sign up is needed to view the answer, it's free)
Wonderful answer; however, I'm curious where you got Pyramid and SQLAlchemy from. I didn't see either of these on Quora or Siftery.
I'm pretty sure I found a trustable source at the time I posted that answer. However, I cannot find that information anywhere on the internet now, so I must assume that either the backend tech has entirely changed, or that they just prefer not to let us know.
Apparently it has now been rewritten in scala: http://making.duolingo.com/rewriting-duolingos-engine-in-scala