The "default.locale" is the default language you want all users to use. But the "webui.supported.locales" is the list of all locales that you are making available from the UI.
After setting "webui.supported.locales" (and restarting Tomcat), you should see the various language options listed in the upper right of the XMLUI. How they are displayed may depend on the theme you are using. But, on our demo site (running Mirage 2), they
are displayed as a dropdown (next to the "Login" link):
http://demo.dspace.org/xmlui/
If you run into more issues, let us know on this list.