The Pentaho Design Studio ships with a single messages.properties which contains all of the base resource strings. If you would like to translate the Design Studio you'll have to find the pentaho-action-editor.jar in the plugins\org.pentaho.workbench.editors.actionsequence_x.x.x.x directory of your Design Studio (Eclipse) installation. Extract this jar using an unzip utility into a working directory.
Navigate into org/pentaho/workbench/messages. Create a .properties file for your locale following the naming conventions outlined in the General Concepts section. So if you have German .properties create a messages_de.properties. For each resource you want to localize copy the key from the base messages.properties into the locale specific properties file and provide the translation for the value (right hand side).
Zip the contents of your working directory and naming the archive pentaho-action-editor.jar, and copy the new jar back into the plug-in directory.