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/views/shared/_plan_change_info.html.erb

@@ -2,11 +2,11 @@
 <% lines = billing_plan_change_info_lines(subscription: subscription) %>
 <div class="plan-change-info-wrap">
   <details class="plan-change-info">
-    <summary class="plan-change-info__trigger" aria-label="Info cambio piano e fatturazione" title="Info cambio piano e fatturazione">
+    <summary class="plan-change-info__trigger" aria-label="<%= t("billing.plan_change_info.trigger_label") %>" title="<%= t("billing.plan_change_info.trigger_label") %>">
       <i class="fa-solid fa-circle-info" aria-hidden="true"></i>
     </summary>
     <div class="plan-change-info__box" role="note">
-      <p class="plan-change-info__title">Cambio piano e fatturazione</p>
+      <p class="plan-change-info__title"><%= t("billing.plan_change_info.title") %></p>
       <ul>
         <% lines.each do |line| %>
           <li><%= line %></li>