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.
Change the user language on the book overview.
Change the user language on the book configuration, this change will apply to all bkper.
Bkper is available in:
Bkper posts and queries can be done in other languages, even allowing to mix LTR with RTL languages.