The message files are located in the source at:
ZimbraWebClient/WebRoot/
js/zimbraMail/config/msgs/ZmMsg_en.
js
ZimbraWebClient/WebRoot/
js/zimbraAdmin/config/msgs/ZaMsg_en.
js
Ajax/WebRoot/
js/dwt/config/msgs/DwtMsg_en.
js
Ajax/WebRoot/
js/config/msgs/AjxMsg_en.
js
However we've changed this slightly in the next release so better to wait, or you'll have to reformat the files a bt.