FEATURE: new site setting to set locale from cookie for anonymous users. (#18377)

This new hidden default-disabled site setting `set_locale_from_cookie` will set locale from anonymous user's cookie value.

config/site_settings.yml

@@ -123,6 +123,11 @@ basic:
     default: false
     client: true
     validator: "AllowUserLocaleEnabledValidator"
+  set_locale_from_cookie:
+    default: false
+    hidden: true
+    client: true
+    validator: "AllowUserLocaleEnabledValidator"
   support_mixed_text_direction:
     client: true
     default: false