l10n(fr): update French localization

Currently translated at 100.0% (146 of 146 strings)

Translation: TW Power Tools/Extension strings (JSON format)
Translate-URL: https://i18n.avm99963.com/projects/tw-power-tools/extension-strings-json-format/fr/
Change-Id: I341d5017c7abe36bf46e647e173855e3c769ab6b
diff --git a/src/static/_locales/fr/messages.json b/src/static/_locales/fr/messages.json
index a4048e2..523166f 100644
--- a/src/static/_locales/fr/messages.json
+++ b/src/static/_locales/fr/messages.json
@@ -608,5 +608,9 @@
     "inject_perforumstats_chart_communityguides": {
         "message": "Guides de la communauté",
         "description": "Label shown in the legend of the per-forum activity chart."
+    },
+    "options_flattenthreads": {
+        "message": "Afficher un switch toggle pour activer/désactiver la vue imbriquée dans les fils.",
+        "description": "Feature checkbox in the options page"
     }
 }