msgmerge squirrelmail/locale/language/LC_MESSAGES/squirrelmail.po \
squirrelmail/po/squirrelmail.po > \
-squirrelmail/locale/language/LC_MESSAGES/squirrelmail.po
+squirrelmail/locale/language/LC_MESSAGES/squirrelmail.po.new
This should keep all strings that are unchanged and comment out all
strings that are no longer in use. You might want to make a copy