Shall we write "they is pretty" or shall we wonder how many people we are talking about?

We shall wonder about the how many people. This is called the "singular they" and it has been a feature of English for hundreds of years.

https://en.wikipedia.org/wiki/Singular_they

Yes, I'm sure almost everyone reading this was already aware. This doesn't mean that "they are pretty. they are clever. they are ugly. they are good." is a good localization of "ő csinos. ő okos. ő csúnya. ő jó."