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 1612dbc..a6ee5d7 100644
--- a/src/static/_locales/ar/messages.json
+++ b/src/static/_locales/ar/messages.json
@@ -179,6 +179,10 @@
     "message": "تقليل المساحة البيضاء في وحدة التّحكّم و TW.",
     "description": "Feature checkbox in the options page"
   },
+  "options_flattenthreads": {
+    "message": "إظهار زرّ تبديل لتمكين/تعطيل طريقة عرض الرّدود المتداخلة في سلاسل المحادثات.",
+    "description": "Feature checkbox in the options page"
+  },
   "options_save": {
     "message": "حفظ",
     "description": "Button in the options page to save the settings"