We do have a prototype ready, we need some testing but it works with the basic feature that we need it. We will relese in some weeks with some more features.
If i´m not wrong ( i just take a look to one video), Polylang as other translation plugins, works by assign to each page of the book one language. Other translation plugins works for a multisite where 1 book is the original and the other books are the translations. We can not have several original books (if i´m not wrong).
In our situation, We need to have different books and different translations.
I have 5 books, and for each one of the books, i have 10 translations. As we work with books, we need for each book, the same number of pages and some times the same name of the pages (like in my situation), that´s why we can automatice the process. Much more simple than with those plugins.
If somebody have different languages in the same book, Polylang looks like a good option, but not in my case where each book is a different language.