It's possible to use the client GWT CONSTANTS/MESSAGES on the server side in Google App Engine.
You have to use the gwt-i18n-server project and modify it so that it works on GAE.
Copy the files in the org.scb.gwt.web.server.i18n folder in your project.
If you use it, you have to copy the CONSTANTS.properties (and MESSAGES, and other nls files) into the folder war/nls_server. You could also create a (svn) link.
Use it so in your Service or Servlet:
// Internationalized "hello" AppNLS nls = GWTI18N.create(AppNLS.class, "de"); // "de" is the language return nls.hello() + " " + yourName;