Soqotri is a language spoken in Socotra by 71,000 people. An ancient language, it belongs to the South Arabian family of languages, the same group spoken by the pre-Islamic inhabitants of Yemen and Oman. Despite its long history, a script was only devised for Soqotri in 2014, by the Russian Dr. Vitaly Naumkin. Despite the small number of speakers, Soqotri is quite healthy, its speakers are young and not changing to Arabic. Soqotri is a very rich language, it has different words for 'to go' depending on the time of day, and for 'give birth' depending on the animal involved. While perhaps difficult, I think it would be fascinating to have Soqotri on here.
I agree with you,many languages are not on duo lingo as they take a lot of time to create excersizes etc. some languages i think they should add are Hindi or Bengali (not sure if they are already there :P)
It's nice to raise awareness for such languages though, and who knows, maybe if enough people want it, Duolingo might be able to talk with authorities in Yemen/Socotra to get it done. Regarding Hindi and Bengali, Hindi is being made, and an English course for Bengali speakers is being done, this is the normal for courses, first the English course for them is made, later (in some cases much later with people desperately waiting, looking at you Arabic) the course of that language for English speakers. The reality is that courses teaching English get more learners than courses teaching English speakers, and people learning English tend to be more determined.