Lifter + Polylang

***Even if you do not support polylang, the bug seems to be small, so perhaps you can point me to the right direction to solve it.***

Our site is multilingual and now I am creating the first course in the second language.

For translation we have been using Polylang and PoEdit for years with no problems.

I just created the course in the new language (no setting available as to the language of the course on the course backend page as far as I can see) and here is the bug:

1. I change the language with the language switcher to the new language
2. But when I click on the link of the new course to open it and the course page loads, the language of the site changes back to the default language rendering the site unreadable for the foreign user.

WHY is this? How to fix it? Thank you for your help.