http://www.xwiki.org/xwiki/bin/view/DevGuide/InternationalizingApplications
could be an entry point, but I think it need a lot of work as far it
don't reflect the state of the art. For intance, xwiki.jar not longer
exists, you must now look for xwiki-core-1.x-[milestone-y].jar file.
You will find ApplicationResources_es.properties there. I don't know at
what extent strings are parametrized in the whole system.
I am still far from understand how this impressive environment work,
but keep trying :-)