After searching a bit more, my opinion changed a bit. I am fine for using singular they in professional or official communication, but here it would simply be confusing.
See various discussions whether it should or should not be used for Duolingo translations https://www.duolingo.com/comment/21135622/Singular-they-in-English
It is not a simple subject and this course must teach Czech as clearly as possible. Any of he/she/it is always possible so we are not discriminating anyone.
On the same note we cannot accept "he or she" either.
If you find that the official translatio s use "he" too often, you can point us to the numbers and it might get changed to "she" or "it" somewhere.