Marketing Duolingo as a fun web translation game and also as a way to learn to improve automatic translation for your own purposes (and not so much about learning a language).
There are probably many people who don't want to learn a language but who will find the web translations a fun challenge regardless.
Maybe Duolingo could be marketed not just for language learning but also as a fun game for those who don't want to learn a language?
Such a game might even be educational not so much in learning another language but in learning how to improve the output of automatic translation so that you can make better sense of the result.
I suppose you could take the translations from the difficult area and run them through a Google translate type program and have people rate (on a scale of 1-5) how much sense they make.