Jonathan:
Both Dan and I have run into a wall getting the WebUI plugins to work
I haven't been able to get them to properly pick up the text value of the I18n key, it always prints the value.
I've tried definiing the I18n resource a couple of different ways, and putting the prop file a couple of different places
the only thing that worked was putting the key/value inside one of the existing confluence prop files.
which obviously isn't a real solution

Charles:
Yeah, I think that's known-broken. Same reason that uploaded language packs don't work at the moment - we have to hack deep in the bowels of webwork to fix it.