On a production install you can find the message files here:
/opt/zimbra/jakarta-tomcat-5.5.7/webapps/zimbra/
js/ajax/config/msgs/AjxMsg_en.
js
/opt/zimbra/jakarta-tomcat-5.5.7/webapps/zimbra/
js/ajax/dwt/config/msgs/DwtMsg_en.
js
/opt/zimbra/jakarta-tomcat-5.5.7/webapps/zimbra/
js/zimbraAdmin/config/msgs/ZaMsg_en.
js
/opt/zimbra/jakarta-tomcat-5.5.7/webapps/zimbra/
js/zimbraMail/config/msgs/ZmMsg_en.
js