Django 3 docs say (and Django 4 actually removes the feature):

To limit creation of sessions and hence favor some caching strategies, django.views.i18n.set_language() will stop setting the user’s language in the session in Django 4.0. Since Django 2.1, the language is always stored in the LANGUAGE_COOKIE_NAME cookie.

We should adapt feincms/extensions/translation.py as well as the tests that rely on that behaviour?

It's not super-urgent, as the fallback for django1.6 also somewhat covers the dj4 case, but that's more a happy accident than intentional.