Transifex translations are not working as expected, we introduced a change https://github.com/pressbooks/pressbooks-book/pull/1190/files to try to setup automated github sync with transifex but we are dealing with some issues regarding duplicate translation resources and missing translations.
Translation files filenames
Before
languages/es_ES.mo
Now
languages/pressbooks-book-es_ES.mo
Solution
Revert the translation prefix change because the old translations on this plugin are not using a prefix like pressbooks does, you can look at previous commits files doesn't contains a prefix
Problem
Translation files filenames
Before
Now
Solution
After this change