This occurs if the Browser Accept-Language is not one of the languages configured for the cms as active language. And djangos get_language_from_request was called previously with something like SettingsOverride (here) where LANGUAGES is overridden with a language we don't want. Django optimises with a cache in a global object that then will return languages actually not in LANGUAGES.
I have a patch. Pull request will follow.
fixes #1549 endless redirect if browser language is not a cms language