You could specify an arbitrary attribute name to ActiveRecord::Errors#add. Arguably, this is an abuse of the API and may not work in future releases of ActiveRecord.
Alternatively, use ActiveRecord::Errors#add_to_base to add a message to the base object instead of a specific attribute. This gives you control of the full error message.
Finally, you could use a localization plugin to handle translation of error messages in the view.
Ciao, Sheldon.