Is this not a bit too much? Do I have to attend an English course to be able to do the Czech Duolingo? I am not a native English speaker and I have never seen this plural form before. I guess that it is Greek but I think that it is a bit odd that I will fail a translation because of that.
Unfortunately this course is designed for English speakers. Take it as an oportunity to exercise your English as well. I do that myself. It is not such an uncommon word after all.