Here is how I managed to change the language file.
Edit the file /languages/yourlanguagetag/tag.com_biblestudy.ini
In my case it is: /languages/en-GB/en-GB.com_biblestudy.ini
This will affect the site strings. There are some untranslated strings. You can see them by going to the global configuration | System and turning on Debug Language.
To see which strings have not been translated, also turn on System Debug. Look at the Bible Study listing. Below all the SQL queries you will see the language files loaded and also any untranslated strings. This makes it easy to edit that language file.
On the admin side, then. Keep the system in debug mode and open up the study listing page. You will also see the language files used there and untranslated strings.
In my case it is: administrator/language/en-GB/en-GB.com_biblestudy.ini AND administrator/language/en-GB/en-GB.com_biblestudy.menu.ini
The menu.ini file mainly handles the menu items from the Components drop down.
I was able to change strings in my site using this method.
Translated strings will have bullets on either side, untranslated strings will have ?? on both sides.
To add a string, edit that .ini language file and add STRING TO TRANSLATE=Translated String
We are still in need of someone to handle the translations for com_biblestudy - it's just too much for the developers. So anyone willing - HELP!
Tom