Here's a high-level view of how you can do it.  Sorry for the lack of
specifics, but we will eventually have a sample demonstrating this
technique.

What you do is define an abstract class called something like
"Messages", with a bunch of abstract methods returning String, one
method per message you need.  Then you define a bunch of concrete
implementations of the class, like "MessagesEN" which actually return
the text you need for that language in a concrete implementation.
Finally, you use deferred binding to return the correct concrete
subclass for the particular language you need.  This involves using
GWT.create() at the point where you instantiate the Messages class, and
also setting up a bunch of "replace-with/when-type-assignable" rules in
your module to tell the compiler how to pick the right class.
Unfortunately, this process isn't well documented yet, so you'll have
to just look at what we're doing in UserAgent.gwt.xml and DOM.gwt.xml
and follow it.  Except you'll need to get the browser's preferred
language instead of its User Agent.

This should be a lot easier when we have a good sample.  We're also
looking at ways to do all this automatically from properties files.

Scott Blum
GWT Engineer