[1.7.x] Fixed #24503 -- Corrected docs on how LANGUAGE_CODE works as a fallback.

* If the locale middleware isn't in use, it decides which translation is served

  to all users.

* If the locale middleware is active, it provides the fallback translation when

  no translation exist for a given literal to the user's preferred language.

* If the locale middleware is active, it provides a fallback language in case

  the user's preferred language can't be determined or is not supported by the

  Web site.

See :ref:`how-django-discovers-language-preference` for more details.