have a language definitions file for each language - then include the current language file
ie langfrench.php
define("HELLO", "Bonjour");
define("GOODBYE", "Au revoir");
ie langenglish.php
define("HELLO", "Hello");
define("GOODBYE", "Goodbye");
ie langdanish.php
define("HELLO", "Hej");
define("GOODBYE", "Hej hej");