display_settings: Fix real-time sync of default language.

We apparently weren't sending to the frontend the data needed to
update the display of the current language.

frontend_tests/node_tests/dispatch.js

@@ -396,6 +396,7 @@ var event_fixtures = {
         type: 'update_display_settings',
         setting_name: 'default_language',
         setting: 'fr',
+        language_name: 'French',
     },
 
     update_display_settings__left_side_userlist: {

static/js/server_events_dispatch.js

@@ -293,6 +293,10 @@ exports.dispatch_normal_event = function dispatch_normal_event(event) {
         if (_.contains(user_display_settings, event.setting_name)) {
             page_params[event.setting_name] = event.setting;
         }
+        if (event.setting_name === 'default_language') {
+            // We additionally need to set the language name.
+            page_params.default_language_name = event.language_name;
+        }
         if (event.setting_name === 'twenty_four_hour_time') {
             // Rerender the whole message list UI
             home_msg_list.rerender();

static/js/settings_display.js

@@ -149,10 +149,7 @@ exports.report_emojiset_change = function () {
 function _update_page() {
     $("#twenty_four_hour_time").prop('checked', page_params.twenty_four_hour_time);
     $("#left_side_userlist").prop('checked', page_params.left_side_userlist);
-    // TODO: We destroy the pencil when doing this, and also there's a
+    // TODO: We destroy the pencil when doing this
-    // problematic mismatch in field name between the event and the
-    // page_params default structure; page_params.default_language
-    // vs. page_params.default_language_name.
     $("#default_language_name").text(page_params.default_language_name);
     $("#translate_emoticons").prop('checked', page_params.translate_emoticons);
     $("#night_mode").prop('checked', page_params.night_mode);

zerver/lib/actions.py

@@ -107,6 +107,7 @@ from zerver.lib.cache import cache_with_key, cache_set, \
 from zerver.decorator import statsd_increment
 from zerver.lib.utils import log_statsd_event, statsd
 from zerver.lib.html_diff import highlight_html_differences
+from zerver.lib.i18n import get_language_name
 from zerver.lib.alert_words import user_alert_words, add_user_alert_words, \
     remove_user_alert_words, set_user_alert_words
 from zerver.lib.notifications import clear_scheduled_emails, \
@@ -2977,6 +2978,10 @@ def do_set_user_display_setting(user_profile: UserProfile,
              'user': user_profile.email,
              'setting_name': setting_name,
              'setting': setting_value}
+    if setting_name == "default_language":
+        assert isinstance(setting_value, str)
+        event['language_name'] = get_language_name(setting_value)
+
     send_event(event, [user_profile.id])
 
     # Updates to the timezone display setting are sent to all users

zerver/tests/test_events.py

@@ -1435,7 +1435,17 @@ class EventsRegisterTest(ZulipTestCase):
                 ('user', check_string),
                 ('setting', validator),
             ])
-            error = schema_checker('events[0]', events[0])
+            language_schema_checker = self.check_events_dict([
+                ('type', equals('update_display_settings')),
+                ('language_name', check_string),
+                ('setting_name', equals(setting_name)),
+                ('user', check_string),
+                ('setting', validator),
+            ])
+            if setting_name == "default_language":
+                error = language_schema_checker('events[0]', events[0])
+            else:
+                error = schema_checker('events[0]', events[0])
             self.assert_on_error(error)
 
             timezone_schema_checker = self.check_events_dict([