Might be that in some cases it's used the joomla internal language.
If they are not left hard coded or in the language file, then they are coming from Joomla.

Not intended. We usually use for the display the language file.

Great to see such support ! 

Is it possible to rename the lang file as fr.php or it.php to have it supported ? Or is it necessary to replace current en.php ?

Moreover : is your component using DB storage for generated ebooks ? I mean, update needs to uninstall previous, right ? Does this lead to need of recreation of all ebooks ?

Thank you very much !