Read the last two paragraphs :)
You can (could, maybe they 'fixed' it by now) get sota LLMs to reproduce entire novels near verbatim.
The idea of giving it parallel texts of those novels in different languages, to train it on translation, is so obvious it'd just be strange if the AI labs didn't do it.
In fact DeepL was doing basically that more than 10 y ago.
I still think there are better tests you could do. Ideally, you would choose a book that was published recently—after the model’s cut-off date—which is considered to be a good translation. But even something like The Girl With the Dragon Tattoo, which is not particularly new and by no means obscure, would be better than a famous work of literature like The Three Musketeers that has many translations.
Even if Fable didn't have Ellsworth's translation, it certainly has the William Barrow translation, which would still get it like 80+% of the way there.
My wife speaks Spanish, I should get her to do some kind of comparison with a Spanish book that doesn't have English translations.