you need to change the localisation of your WP install, which I dont know how to do – and whatever local you choose, cart will automatically choose that language file. I havent done it yet but that seems the way: http://wordpress.org/support/topic/cart66-running-on-another-language
You can hard change the cart.php file, but like changing views folder any plugin updates will wipe it. take out the php stuff and hard code your new words instead