Discovered while testing MDLQA-16996, it might be the same bug as MDL-72882.

On my local site, I had few language packs Portuguese (pt_BR) and Spanish (es), I have setup a course in topics format following the QA test above, but I was annoyed seeing the content of the course in Portuguese but following the testing instructions in English so decided to remove those language packs completely... after returning to the course and going to edit settings page, I see this error:

That error message didn't go away even after purging caches or log out and log in again, note that the course index also hangs in that loading mode and never render the content:

The tester also reported that is not able to navigate between topics using the buttons. I believe they were replaced by the course index? Still that needs to be confirmed and QA instructions updated if that's the case.