If you're trying to use a language other than english, and it's not working, go to:
http://www.squirrelmail.org/download.php
Download the latest file: all_locals-x.x.xx.xx.tar.gz to a temp directory.
Extract it, then run the ./install script that comes with it.
Use the squirrelmail install path of: /var/www/html/squirrelmail