Selecting book language on signup should update book information accordingly

[greatislander]

Prerequisites

• Can you reproduce the behavior you wish to address with all other plugins deactivated?
• Are you running the latest version of Pressbooks and the latest version of WordPress?
• Are you experiencing this issue with one of our built-in root or book themes?
• Did you perform a cursory search to see if your bug or enhancement is already listed?

Description

Selecting a book language on wp-signup.php does not update the corresponding value in Book Information (pb_language).

Steps to Reproduce

1. Create a new book, selecting a language from the "Book Language" menu.
2. Navigate to the Book Information page of your new book.

Expected behavior: The language you chose should be reflected on the Book Information page. (Even better: all languages available in Book Information are shown on wp-signup.php and update Book Information accordingly.)

Actual behavior: The default language (English) is selected.

System Information

Pressbooks: 3.9.6
WordPress: 4.7
PHP: 7.1