I have been answered through another support channel, so I will "auto-answer" the question:
If you need to localize the message in different languages, you should, from the properties panel of the sequence flow, set the message as Custom and then Syncrhonize a Web Project with the BPM project. A new operation module will be created, then you have to manage it from the Web Project with the classic workaround.
It would be nice to have a body templete in the next version.
Thanks