i18n: Add i18n to 'REDACTED' message in push notifications.

When a user has disabled message content in mobile push notifications,
we send a fixed string (currently "REDACTED") as the content of the
notification. Previously, this string was not tagged for translation;
we fix that here.

Additionally, because mobile push notifications are generated in a
queue worker, they do not have the user's language set by the Django
middleware. Our email notifications solve that problem using
`override_language`; we do the same here.

We choose to do override_language in get_message_payload_apns and
get_message_payload_gcm, rather than the caller, in order to be
consistent with tests.

Tested end-to-end by tabbott by setting a translation for "REDACTED"
manually in German.

Fixes #18713.

zerver/lib/push_notifications.py

@@ -14,6 +14,7 @@ from django.db import IntegrityError, transaction
 from django.db.models import F
 from django.utils.timezone import now as timezone_now
 from django.utils.translation import gettext as _
+from django.utils.translation import override as override_language
 
 from zerver.decorator import statsd_increment
 from zerver.lib.avatar import absolute_avatar_url
@@ -629,7 +630,7 @@ def get_mobile_push_content(rendered_content: str) -> str:
         return plain_text
 
     if settings.PUSH_NOTIFICATION_REDACT_CONTENT:
-        return "***REDACTED***"
+        return "***" + _("REDACTED") + "***"
 
     elem = lxml.html.fromstring(rendered_content)
     plain_text = process(elem)
@@ -744,6 +745,7 @@ def get_message_payload_apns(user_profile: UserProfile, message: Message) -> Dic
     )
 
     assert message.rendered_content is not None
+    with override_language(user_profile.default_language):
         content, _ = truncate_content(get_mobile_push_content(message.rendered_content))
         apns_data = {
             "alert": {
@@ -765,6 +767,7 @@ def get_message_payload_gcm(
     """A `message` payload + options, for Android via GCM/FCM."""
     data = get_message_payload(user_profile, message)
     assert message.rendered_content is not None
+    with override_language(user_profile.default_language):
         content, truncated = truncate_content(get_mobile_push_content(message.rendered_content))
         data.update(
             event="message",