test_classes.py: Add activation to default language.

Running the backend tests with a high number of processes can cause
unexpected errors with language changes.  When certain tests that change
the default language, (without explicitly overriding the teardown method
to reset the default language), interleave with other tests that are
expecting the language to be in English, discrepancies arise.

This fixes a common nondeterministic test failure with high levels of
parallelization.

zerver/lib/test_classes.py

@@ -16,6 +16,7 @@ from django.db.migrations.executor import MigrationExecutor
 from django.db import connection
 from django.db.utils import IntegrityError
 from django.http import HttpRequest
+from django.utils import translation
 
 from two_factor.models import PhoneDevice
 from zerver.lib.initial_password import initial_password
@@ -100,6 +101,7 @@ class ZulipTestCase(TestCase):
         clear_client_event_queues_for_testing()
         clear_supported_auth_backends_cache()
         flush_per_request_caches()
+        translation.activate(settings.LANGUAGE_CODE)
 
     '''
     WRAPPER_COMMENT: