How are new courses chosen?

Does anyone know how Duolingo chooses the courses that get to go into the incubator? I often see people say they have applied to make course x, but never hear back from Duolingo. Must there be a certain amount of people willing to contribute? Can there only ever be x amount of languages in the incubator at a time?

I know that Duolingo wants to maintain high quality courses, but it seems strange that when there is a significant demand for a language, like Latin, for example, they don't at least put it in the incubator so people will know it is coming.