[1.10.x] Fixed #15143 -- Doc'd how to set the language for the test client.

Backport of 3c447b108a from master

docs/topics/i18n/translation.txt

@@ -1950,6 +1950,8 @@ Here's example HTML template code:
 In this example, Django looks up the URL of the page to which the user will be
 redirected in the ``redirect_to`` context variable.
 
+.. _explicitly-setting-the-active-language:
+
 Explicitly setting the active language
 --------------------------------------
 

docs/topics/testing/tools.txt

@@ -574,6 +574,44 @@ can access these properties as part of a test condition.
             session['somekey'] = 'test'
             session.save()
 
+Setting the language
+--------------------
+
+When testing applications that support internationalization and localization,
+you might want to set the language for a test client request. The method for
+doing so depends on whether or not the
+:class:`~django.middleware.locale.LocaleMiddleware` is enabled.
+
+If the middleware is enabled, the language can be set by creating a cookie with
+a name of :setting:`LANGUAGE_COOKIE_NAME` and a value of the language code::
+
+    from django.conf import settings
+
+    def test_language_using_cookie(self):
+        self.client.cookies.load({settings.LANGUAGE_COOKIE_NAME: 'fr'})
+        response = self.client.get('/')
+        self.assertEqual(response.content, b"Bienvenue sur mon site.")
+
+or by including the ``Accept-Language`` HTTP header in the request::
+
+    def test_language_using_header(self):
+        response = self.client.get('/', HTTP_ACCEPT_LANGUAGE='fr')
+        self.assertEqual(response.content, b"Bienvenue sur mon site.")
+
+More details are in :ref:`how-django-discovers-language-preference`.
+
+If the middleware isn't enabled, the active language may be set using
+:func:`.translation.override`::
+
+    from django.utils import translation
+
+    def test_language_using_override(self):
+        with translation.override('fr'):
+            response = self.client.get('/')
+        self.assertEqual(response.content, b"Bienvenue sur mon site.")
+
+More details are in :ref:`explicitly-setting-the-active-language`.
+
 Example
 -------