Pontoon: Update Arabic (ar) localization of TW Power Tools

Co-authored-by: Medelinor <eloueryaghlymohamed@gmail.com>
diff --git a/src/static/_locales/ar/messages.json b/src/static/_locales/ar/messages.json
index a5cf6e1..62b2999 100644
--- a/src/static/_locales/ar/messages.json
+++ b/src/static/_locales/ar/messages.json
@@ -283,6 +283,10 @@
     "message": "لأسباب فنية، لا يمكننا تحميل الصور الرمزية للمواضيع المنشورة في المنتديات الخاصة.",
     "description": "Helper text which appears when hovering an icon next to a thread, to explain its meaning."
   },
+  "inject_threadlistavatars_invisible_thread_indicator_label": {
+    "message": "لأسباب فنية، لا يمكننا تحميل الصّور الرّمزية لسلاسل المحادثات غير المرئية للمستخدمين المجهولين.",
+    "description": "Helper text which appears when hovering an icon next to a thread, to explain its meaning."
+  },
   "inject_workflows_menubtn": {
     "message": "تشغيل سير عمل...",
     "description": "Tooltip of the icon shown above a thread or in thread lists when selecting multiple threads in the Community Console which lets the user show a menu with the worklofws they can run."