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

Co-authored-by: Medelinor <eloueryaghlymohamed@gmail.com>
diff --git a/src/lit-locales/source/ar.xlf b/src/lit-locales/source/ar.xlf
index 84da080..893f73a 100644
--- a/src/lit-locales/source/ar.xlf
+++ b/src/lit-locales/source/ar.xlf
@@ -19,12 +19,13 @@
       </trans-unit>
       <trans-unit id="s6b0ed0c74667d28c" xml:space="preserve">
   <source><x id="0" equiv-text="${authorName}"/> said:</source>
-      <target><x id="0" equiv-text="${authorName}"/> قال:</target>
+        <target><x id="0" equiv-text="${authorName}"/> قال:</target>
       </trans-unit>
-<trans-unit id="s98a0ccd6f423421f">
+      <trans-unit id="s98a0ccd6f423421f" xml:space="preserve">
   <source>Nested view</source>
-  <note from="lit-localize">Label for the switch which lets users enable/disable the nested view in a thread.</note>
-</trans-unit>
+        <target>عَرض متداخِل</target>
+        <note from="lit-localize">Label for the switch which lets users enable/disable the nested view in a thread.</note>
+      </trans-unit>
     </body>
   </file>
 </xliff>
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"