Thanks for your replies.
As I'm using spring with jbpm, I think I will looking arround with the resource bundle of spring, and I thinking about velocity to générate content as string for subject and content with an event listener (start event of mail task) and putting all i18n resources into some resource bundle.
An other option is to extend MailProducer and manage all resources within spring.
I don't know if it's an optimised way but it seem to be not too horrible to implements.
Regards