I suggest a few additions to aid with books in other languages:
A UI option to override the "lang" attribute for the current book.
Many books don't set their language correctly, preventing the other features from working.
A UI option to override the "hyphens" CSS property for the current book to toggle automatic hyphenation.
Languages with longer words make more aggressive hyphenation desirable, but ultimately it's a matter of taste that should be left to the user.
Use the correct dictionary for the set language, if available.
I suggest a few additions to aid with books in other languages: