Ensure that flash message is shown in the updated language

app/controllers/preferences_controller.rb

@@ -22,7 +22,8 @@ class PreferencesController < ApplicationController
                                       params[:user][:preferred_editor]
                                     end
     if current_user.save
-      flash[:notice] = t ".success"
+      # Use a partial so that it is rendered during the next page load in the correct language.
+      flash[:notice] = { :partial => "preferences/update_success_flash" }
       redirect_to preferences_path
     else
       flash[:error] = t ".failure"

app/views/preferences/_update_success_flash.html.erb

@@ -0,0 +1 @@
+<%= t ".message" %>

config/locales/en.yml

@@ -1655,8 +1655,9 @@ en:
       save: Update Preferences
       cancel: Cancel
     update:
-      success: Preferences updated.
       failure: Couldn't update preferences.
+    update_success_flash:
+      message: Preferences updated.
   profiles:
     edit:
       title: Edit Profile

test/system/preferences_test.rb

@@ -0,0 +1,23 @@
+require "application_system_test_case"
+
+class PreferencesTest < ApplicationSystemTestCase
+  def test_flash_message_shows_in_original_language
+    sign_in_as(create(:user))
+
+    visit edit_preferences_path
+    click_on "Update Preferences"
+
+    assert page.has_content?("Preferences updated")
+  end
+
+  def test_flash_message_shows_in_new_language
+    sign_in_as(create(:user))
+
+    visit edit_preferences_path
+    fill_in "Preferred Languages", :with => "fr"
+    click_on "Update Preferences"
+
+    # TODO: enable with french translation when that's available
+    # assert page.has_content?("Préférences mises à jour") # rubocop:disable Style/AsciiComments
+  end
+end