Sponsor a language
If Duolingo is looking to raise a little money they should have a option to sponsor/support a language. People could choose do donate $2-$3/month for each language they want to support. Duolingo could display the number of supporters for each language or maybe show people as being supporters on the forums in some way.
I would love to donate money regularly like this. Not just for languages that I want to learn - also for programs that are urgently needed.
For example: a Turkish course for Arabic speakers. There are over 3 million registered Syrian refugees in Turkey today, who are struggling to make a living in an environment that speaks a very different language. A Duolingo course would make it much easier for them to find jobs & housing, to deal with local bureaucracy, etc.
I'd also love to see a general "language preservation fund" on the site, that would support an ever-expanding number of languages in danger of dying out. That way, even if a project is less popular or publicized (e.g., if it belongs to a small group of people), it still has a chance of getting resources if enough volunteers offer their services.
There is a way to do this. And it is the reason why some are PLUS users.
As they wish to support the Duolingo mission
as a Duolingo Plus subscriber, you support our mission to keep education free for millions around the world. ref
To learn more, click https://support.duolingo.com/hc/en-us/articles/115004068043-What-is-Duolingo-Plus-
edited. Yes, it does not allow you to support an individual language. Which is what I also read into your comment. It will though support all the languages Duolingo is able to support.
Many languages, while not yet in the incubator, may have resources being collected, behind the scenes in Duolingo. Duolingo does strive to support as many languages as it can.
People may also like to visit:
And/or perhaps consider creating forum posts that assist in learning the language you would like to see Duolingo take on. These will also be useful when eventually Duo is able to take on your desired target language (I know I am being hopeful. Though from what I have observed, Duo would love to be able to support more languages.) IMO.
Perhaps also check out [LTI] Language resources not (yet) on Duolingo for other ideas.