Particularly I was having the problem with the new client-side
validations shipped in GWT 2.4, I didn't make it clear. Maybe there
was some generation issue that is preventing it from working (this is
still beta).
However I could finally solve it by overriding
org.hibernate.validator.ValidationMessages.properties in my project. I
know it's not exactly the most beautiful thing in the world but it
worked for me.
Thanks for your reply, Kevin.