We only have English and Spanish translations on our website. I can easily translate each book page, but there doesn't seem to be a way to translate the book tree navigation. Is there a way to translate the book tree navigation so the links will display translated text and link to the translated pages?
I found this that outlines the path to doing this with menus in general: https://www.drupal.org/node/2182497
After testing it appears that this does NOT work with the Book module menus. All configuration options I have tried thus far have not been successful and I'm guessing the only options left are probably a custom code alter of the book_navigation or a custom book menu all together.
Those steps would included either using the book_navigation preprocess hook: function template_preprocess_book_navigation https://api.drupal.org/api/drupal/core%21modules%21book%21book.module/function/template_preprocess_book_navigation/8.2.x
and/or the twig template that renders the menu: book-navigation.html.twig https://api.drupal.org/api/drupal/core%21modules%21book%21templates%21book-navigation.html.twig/8.2.x
Hopefully those are a good start.