You can localize a Pentaho Report and keep the property files external. This enables you to share property files among multiple PRPT reports, which minimizes the files you need to maintain. To use external message bundles (.properties files), define the key/value pairs, but place the bundles on the classpath for the report engine to find.
If you are in Pentaho Report Designer, add the files to the [PRD Install]/resources directory. For them to be recognized in the Pentaho Webapp, put the files in the pentaho/WEB-INF/classes directory.