I'm asking because I tried (angain an again ...) to install objavi and an error was about lang, I think it is for language.
Then I searched in Booktype all about "language" and I found this Button with no effects. And at right of this button, I have only one choice : "unkown language".
I have to say : all my tests are in virtual machines (created with lxc) and no language in all I created (no need until now).
This is as option which should tell Objavi to use options of "Right To Left" writing and organize books according to language on the Booktype side. It assumes it is english by default and you can ignore this option for now. Devel branch does initially populate database with languages but for older options it has to be done manually.
I didn't write the correct message somewhere and it was one of the new ideas to have a working objavi. Since I destroyed the guilty virtual machine...
I followed this path because I don't see English or other language in my Booktype, created with Aleksandar Erkalovi's script in lxc containers. I don't have English by default.