Completa i18n IT/EN/FR/DE/ES su sito pubblico, area autenticata e admin.

Tutte le view marketing, legali, viewer, dashboard e admin usano t(); mailer utente-facing e flash localizzati; admin con LocaleResolver e selettore lingua.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/controllers/admin/youtube_controller.rb

@@ -2,7 +2,7 @@ module Admin
   class YoutubeController < Admin::BaseController
     def platform
       if ENV["YOUTUBE_CLIENT_ID"].blank?
-        redirect_to admin_root_path, alert: "Configura YOUTUBE_CLIENT_ID e YOUTUBE_CLIENT_SECRET in .env"
+        redirect_to admin_root_path, alert: t("admin.flash.youtube_not_configured")
         return
       end