I'd like to suggest Latin as another language to add to Duolingo. Spanish and English as well as a couple of other languages have their roots in Latin. I think it would be a good learning experience for those interested in the history of our main languages as well as those who like learning some of the old languages (Like me.).
There is more Latin on the web and more Latin speakers than one might think. Latin has continued to be (at least in part) a conversational language within the institutions of the Catholic church, and in recent years there has been quite a renaissance in teaching Latin in schools as a spoken language. There is a Latin Wikipedia, and quite a lot of other web material in Latin, if you look for it. Whether including it on Duolingo would fit their business model is another matter entirely, of course. But the Duolingo format could work well for Latin, I think, provided that adequate grammatical explanations were provided (which is not Duolingo's strength in other languages).
While this may be a good idea in itself, it does not fit well into Duolingo's business concept, as there hardly is any latin text that has not been translated into other languages and there are hardly any actual speakers, and since Duolingo is based around translating the web they would be at a loss here.
Yes, English is descended from the Germanic languages, which is evident in its basic vocabulary and in aspects of its grammar (although this has been heavily modified). But a very large proportion of its vocabulary is derived from Latin, either directly, or via French. The huge influx of French-derived vocabulary, beginning at the time of the Norman Conquest, is evident in Chaucer. The most helpful comparison here is between the still almost entirely Germanic Old English of Beowulf (which native speakers of English can't read without special training) and the much more modern looking Middle English of Chaucer, which a modern native speaker can at least recognize as English, even if it takes special training and dictionaries to learn to read it fluently.
I concur, and it would be quite useful for seminarians or others from traditions where Latin texts are widely read in study. European history, Catholic religion, and other fields would benefit greatly from a Latin duolingo. I would be one of the first to start that program.