UX: Prevent users from wondering why backfilling is not happening

Author: nattsw

config/locales/server.en.yml

@@ -20,6 +20,7 @@ en:
     experimental_anon_language_switcher: "Enable experimental language switcher for anonymous users. This will allow anonymous users to switch between translated versions of Discourse and user-contributed content in topics."
     errors:
       set_locale_cookie_requirements: "The experimental language switcher for anonymous users requires the `set locale from cookie` site setting to be enabled."
+      needs_nonzero_backfill: "Automatic language translation requires the 'automatic_translation_backfill_maximum_translations_per_hour' hidden setting to be a non-zero value. Please approach your site administrator to increase this limit."
     experimental_inline_translation: "Enable experimental inline translation feature. This replaces existing parallel translation, allowing site visitors with a non-default locale to view content in their language."
     automatic_translation_target_languages: "The languages to automatically translate user content (posts, topics) to. If empty, no languages will be automatically translated."
   translator:

config/settings.yml

@@ -16,10 +16,12 @@ discourse_translator:
     validator: "DiscourseTranslator::TranslatorSelectionValidator"
   automatic_translation_target_languages:
     default: ""
+    client: true
     type: list
     list_type: named
-    choices: "DiscourseTranslator::TranslatableLanguagesSetting.values"
     allow_any: false
+    choices: "DiscourseTranslator::TranslatableLanguagesSetting.values"
+    validator: "DiscourseTranslator::TranslatableLanguagesValidator"
   automatic_translation_backfill_maximum_translations_per_hour:
     default: 0
     client: false

lib/discourse_translator/translatable_languages_validator.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module DiscourseTranslator
+  class TranslatableLanguagesValidator
+    def initialize(opts = {})
+      @opts = opts
+    end
+
+    def valid_value?(val)
+      return true if val.blank?
+
+      SiteSetting.automatic_translation_backfill_maximum_translations_per_hour > 0
+    end
+
+    def error_message
+      I18n.t("site_settings.errors.needs_nonzero_backfill")
+    end
+  end
+end

spec/system/site_settings_validation_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+RSpec.describe "Translation settings", type: :system do
+  fab!(:admin)
+
+  before do
+    sign_in(admin)
+    SiteSetting.translator_enabled = true
+  end
+
+  it "warns when automatic_translation_target_languages is being set but backfill limit is 0" do
+    visit(
+      "/admin/plugins/discourse-translator/settings?filter=automatic%20translation%20target%20languages",
+    )
+
+    setting =
+      PageObjects::Components::SelectKit.new(
+        "[data-setting='automatic_translation_target_languages'] .select-kit",
+      )
+    setting.expand
+    setting.select_row_by_value("ja")
+
+    page.find(".setting-controls button.ok").click()
+
+    expect(page).to have_content(I18n.t("site_settings.errors.needs_nonzero_backfill"))
+  end
+end