Two suggestions: (1) provide computer opponents with rubber banding to motivate learners; (2) give out medals to the top three credit earners each week among you, your friends, and your computer opponents.
Rubber banding explained here: http://www.gamasutra.com/blogs/DaveMark/20100602/87445/RubberBanding_as_a_Design_Requirement.php
Note that weekly medals can work without any friends; you would compete against just computer opponents.
The article comments that obvious rubber banding doesn't work well. If you are talking about a computer opponent that gets points on duolingo, it is clear that the computer opponent isn't actually learning the language. So it will be pretty obvious that the computer opponent will just get a few more or less points than you.
Now if you're talking about creating some head to head language games, then a computer opponent would work well.