"Il a des dizaines de livres anglais."
"Douzaine" is dozen, while "dizaine" means "about ten," right? So, the more literal translation would be "He has tens of English books." Not a very common thing to say in English, but not unheard of. I'm think the Duolingo translation is probably better here, but I'm not 100% sure.
"English books", if said in (at least, american) english, would imply "textbooks regarding the use and study of the English language" not "books of all topics written in english" which the sentence seems to be saying. I don't know why it rejected my answer, "tens of books in english"
I think the meaning in the French is "books from England" or "books from Britain".
The literal translation of the French to the English does not translate to what you have said, "tens of books in english". It doesn't mean that. It simply says "English books", nothing more.
"To have tens of objects" means that the subject has so many such objects that if you were to group all the objects into groups of ten, there would be exactly or more than two such groups. That is, if you were to divide to the number of objects by ten, the resultant number is greater than or equal to two. It is most often used to say that the subject has many such objects, regardless of whether the subject actually has that amount of objects.
I agree that "dozen" is more often used to express a similar notion of a large quantity, but here the literal translation requires the use of "tens".
Have you ever heard someone say "tens of hundreds of dollars", or something similar? That might help you.