I know that the Spanish language course is/will be available, but what about the other major languages?
I'm the lead contributor for Duolingo's Esperanto course for English speakers as well as initiating the Esperanto course for Spanish speakers, so I believe that qualifies me to answer this. Duolingo uses three criteria for deciding whether a new course should be launched:
- How much demand is there for this course?
- Are there enough high-quality bilingual candidates to create this course?
- Are there any technical limitations to creating this language pair?
As for #3, it's already been proven that the system can handle teaching an Esperanto course. Duolingo is actually amazingly well suited for Esperanto, since it teaches by induction and given the few exceptions Esperanto has, induction works incredibly well.
As for #1 and #2, this will most likely determine which order new Esperanto courses are launched on Duolingo. After many public polls, Esperanto was the most demanded language twice, so there was no way Duolingo could possibly ignore it any longer, so then they started sifting through the candidates to choose their team. As for Spanish, there are so many Spanish speakers learning languages on Duolingo that this course was a no-brainer.
Now, looking at the number of people learning languages on Duolingo, the next likely candidates for an Esperanto course in my opinion would be Chinese, French, German, Portuguese and Russian. I believe Russian is the best option, because there are already many Duolingo courses for Russians, plus this would diversify the Duolingo community from being concentrated only in the Americas. Plus, the Russian Esperanto community is very strong with many excellent speakers and teachers, thus ensuring the course will be of very high quality.
But, in the end, it is the Duolingo community team, which will have to decide whether it believes that another Esperanto course would be beneficial for the site. I certainly hope so!
The Duolingo Incubator is where courses that are "under development" are listed. Unless you get a quote from a Duolingo employee, the Incubator serves as the first public notification of a future course. Duolingo only lists courses here once it has selected contributors for the course and approved development of it.
At the current time (May 2018), the only courses teaching Esperanto are Esperanto for English, for Spanish, and for Portuguese:
The "phase" of each course indicates where it lies in its development. Esperanto for English and Esperanto for Spanish have graduated (phase 3), while Esperanto for Portuguese is expected to be released to beta (phase 2) soon.
The way this traditionally works in Duolingo is that lots of people apply to create courses but do not get any feedback until Duolingo decides the time is ripe. Normally they first approve English for speakers of X, then X for speakers of English, and then, one after another, various other combinations that involve X and languages which are either very popular in the Western world (essentially Duolingo's initial languages: English, French, Spanish, Portuguese, German) or have a special relationship with X.
Since Esperanto has almost no native speakers, they made an exception for it and we will probably have to wait quite a while for an English course for Esperanto speakers. (Normally, learning these 'reverse' courses is a good next step after finishing a language in Duolingo.)
In Duolingo forums there has been speculation that the next course after Esperanto from English will be either Esperanto from Portuguese or Esperanto from German. However, the team has said that after finishing Esperanto from Spanish they will extend Esperanto from English. This might well delay the other courses even further.