Learn the language Duolingo is written in, the Duolingo way (almost)!
As I'm sure many of you know already, Duolingo has recently been rewritten in the programming language Scala, making it a lot faster than it used to be.
If you are one of those people whose love of languages extends to programming languages, you might like this site:
It is structured similarly to Duolingo's exercises, in that you have lessons with exercises where you have to type short pieces of code, and execute them to see if they are correct.
A word of warning, though: the site is geared for intermediate and advanced learners already familiar with programming concepts. And you should probably begin with the "Scala Tutorial" course.