At the current time of writing this post there is an unanswered issue in Drupal.org concerning the Webform Localization module. It seems that if you have enabled the Show Localization by String Translation option you can't rewrite the Confirmation message any more. I didn't have the time to look more thoroughly into it but as a fallback-mechanism, all I had to do was to:
- Uncheck the Expose webform component strings suitable for translation option
- Save the Form settings of the webform
- Rewrite your confirmation message and save.
- Recheck the Expose webform component strings suitable for translation option.