Fixed #18365 - Added a reminder of the context processor required for the set_language view.

Thanks Nick Martini for the patch.

docs/topics/i18n/translation.txt

@@ -1251,6 +1251,11 @@ As a convenience, Django comes with a view, :func:`django.views.i18n.set_languag
 that sets a user's language preference and redirects to a given URL or, by default,
 back to the previous page.
 
+Make sure that the following item is in your
+:setting:`TEMPLATE_CONTEXT_PROCESSORS` list in your settings file::
+
+    'django.core.context_processors.i18n'
+
 Activate this view by adding the following line to your URLconf::
 
     (r'^i18n/', include('django.conf.urls.i18n')),