Setting the user language is done by each individual user. The chosen user language is applied system wide to all this individuals' books. Two different users might work on a shared book each having a different user language.
On the book overview.
Change the user language on the book overview.
Choose your language.
On the book configuration
Change the user language on the book configuration, this change will apply to all Bkper.
Bkper posts and queries can be done in other languages, even allowing to mix LTR with RTL languages.