Speaking as a total ignorant from a coding perspective. But I guess that wouldn’t be the hard part, considering that most of Duolinguo is just boxes and text inputs. How difficult it is to create a database of competent linguists with an efficient training who can progressively enhance your understanding of languages?
The technology is probably the smallest part of the problem. Most of it will be getting it critical mass of users, and expanding that user base, cross multiple languages. So that’s advertising, politics, social networking, promotion.
Yeah and their staff have been very dogged about promotion in language communities, including crowdsourcing the content (i.e. getting their users to produce it for free)