Capture the language preference set by an anonymous user when the register or login

[LEARNER-168]

common/djangoapps/student/tests/factories.py

@@ -84,9 +84,11 @@ class UserFactory(DjangoModelFactory):
         model = User
         django_get_or_create = ('email', 'username')
 
+    _DEFAULT_PASSWORD = 'test'
+
     username = factory.Sequence(u'robot{0}'.format)
     email = factory.Sequence(u'robot+test+{0}@edx.org'.format)
-    password = factory.PostGenerationMethodCall('set_password', 'test')
+    password = factory.PostGenerationMethodCall('set_password', _DEFAULT_PASSWORD)
     first_name = factory.Sequence(u'Robot{0}'.format)
     last_name = 'Test'
     is_staff = False