Exception Notification overrides rescue action in public and for this reason Rails edge localized rescue (i.e. rendering public/500.en.html) does not work properly.
I've created a patch a long time ago, when localized rescue was still a plugin but I hope it gets the proper attention now since localized rescue is part of rails core.
The patch is here:
Of if you prefer, you can pull from my repository:
http://github.com/josevalim/exception_notification/tree/master
Regards,
José Valim.