So, how many people need to become interested in appearing of some course in Incubator so Duolingo add it? It's quite anoying that in learning Czeh language I'm making mistakes not because I don't understand the Czeh senteces but because of english.
We may have already talked about this. widle is right, the biggest hurdle for pretty much any new course is Duolingo's own lack of desire to add it. The most recent additions have been about protecting endangered languages, but for Slavic languages, the expected user count would govern the decision. It is educational to compare the user counts here.
It is tough to make the case for another course for Ukrainian speakers when you consider that their English course is less attended than English for Czech speakers despite the much larger Ukrainian population. Not even lumping in the whole of Slovakia as if they were Czech speakers can come anywhere close to explaining the difference.
If there is another course for Slavic speakers, I would expect English for Bulgarian or Serbian speakers to come first. And should there ever come an inter-Slavic course, my lingots are on Russian for Polish speakers. I do not expect to see another course involving Czech or Ukrainian speakers before Duolingo becomes a very different company.
Well, because of lazy people who don't study foreign languages (or just don't know about Duolingo), I will have to learn two languages at once ... I hope this won't stop me by the way, when I'll pass the express course of Czech and pass the exam on B -2 for admission to university (because going now to ordinary courses just not enough time) ... Well, as it says: The more languages you know, the more you are a man!
But to edit a course (I suppose that's what you want) you don't need new incubators. And Czech incubators are really serious and dedicated people, specially Nueby. Cross your fingers for him not to see this, his a lot of work in the course, he diserves some respect. Just be patient. :)