This happens with content such as pages or books that are translated using the multi-lang filter or the multi-lang v2 filter.

To reproduce:

• Create a course using multi-lang in the name of the course, name of at least one section (English and Spanish)
• Create a page using multi-lang in its title, description and content
• Force a student profile language to English on the Moodle site
• Open the app, connect to the site with the same student account and change the app language to Spanish
  Confirm that:
• The course name course section and page name are displayed in Spanish
  but
• The page content is always displayed in English

Originally reported via MOBILE-4629, that site allows public sign-up so we can use it to check the books in the "Introduction to Triathlon" course.

In order to address this issue we should append the app current language as a parameter to any request to pluginfile.php or tokenpluginfile.php

For example:
tokenpluginfile.php/f9d6cf8e134bc9e671086eac5d5063d8/47089/mod_book/chapter/26994/index.html?offline=1&lang=es

Please notice also that once the app is downloaded (for example in Spanish) following access will always display the page in Spanish, even if the app is configured to use English as it was already downloaded.