Hi Luis, this is truly awesome, in the traditional sense.
I think it deserves a Nobel Prize for the concept alone, even before it fulfills its potential to change the world.
If you never include Esperanto, Duolingo will give even poor people a chance to learn a economically powerful language (providing that they have hundreds of hours to study and the same amount of internet connection time), and will show the economically powerful that non-English-speakers exist and think and create.
If you do include Esperanto, which is designed to take a fraction of the time to learn- no more than one sixth compared to another European language, then you will both include more poor people in your gift and include more of the wealthy but distracted, who are unsure of whether they can learn another language, or whether the effort is justified.
Esperanto is a linguistic middle-ground which provides the opportunity for all global citizens to communicate on a equal basis.
Besides that, there is also good evidence that learning Esperanto first facilitates and motivates the learning of subsequent languages.
I hope that you will give this serious consideration and would be happy to coordinate collaboration to include Esperanto in your list.
Thanks again for your gifts to the world to date, and for the inspiration they provide by showing what can be done with better thinking.

For the Nobel Prize he’d just need to get the millions of Farmvillians to plant and tend real crops and FeedEx and others to ship them for free. It would be the end of world hunger.