If you set 'force language' for an activity (say a Page) then write all components of that activity in the language, this adds a language tag to the root of the activity page, but does not add tags around parts of that activity text that appear on other pages, mostly the course page.
For example, say you force Page language to 'fr', and set Name, Description, and Content to:
Je suis un nom
Je suis une description
Je suis le contenu de la page
When viewing the page itself, there will be a lang="fr" tag on the page, so all these components will correctly read in screen readers.
On the course page, the name and (if set) description will be displayed, but these don't have a lang tag, so a screenreader might read them out in the wrong language.
I think there are other problems around language tagging for course elements (obviously it's ok if you explicitly use the language filters), for example I'm not even sure we really have the concept of a default language for a course so if a user changes language it won't work for course elements thern either. So this might not be a very easy problem to solve in general, but this specific case became a bit more obvious as a result of the new (in 4.1) feature to force activity language.
Note - I'm filing this but I don't think we can fix it any time soon, the product owners just identified the problem and asked me to record it.