The English answer needs an apostrophe, as in the way it is written here, otherwise it is incorrect
I noticed that the apostrophe was missing, so I went to keyboard mode and answered . It was marked as incorrect.
I, too, have had trouble with some of the tile-choice questions. When the possessor is plural, there are no tiles with apostrophes, and we aren't given the correct tiles for the "the x of the y" construction.
It seems like there was a small typo in the incubator. It's been fixed, thank you for the comment!
I too had "the dress of the girls" marked as incorrect, even though this is a grammatically correct translation.
I wrote 'the girls' dress' for which I was marked 'almost correct', but then this was given as an alternative correct answer. Incidentally, I have never in other contexts come across a plural possessive for this sort of word without an inverted comma. I don't think this is British usage at least.
The inverted comma is properly called an apostrophe, but otherwise you are perfectly right, and it isn't a British or otherwise regional usage. I went to school in the US, and as I learned grammar, the apostrophe is mandatory for any possessive noun.
There is an apostrophe missing on the multiple-choice answer on Android. "Girls' " is the correct English translation of κοριτσιών - without the apostrophe it is just κορίτσια.
I wrote "The girls' dress" and received the answer that I have a typo and the correct answer is "The girls dress" which is incorrect using android.
των κοριτσιών is genitive plural. The correct translation is The dress of the girls, which is added in the Incubator :)
The dress of the girl = το φόρεμα [του κοριτσιού/της κοπέλας]