Lots of applications simply initialize a $lang array in an included page; the default is (in English-speaking countries) English, and a session variable, if set by the user selects a different array to be used.
So, the welcome page might have:
<?php
session_start();
include 'lang.php';
echo $lang['welcome'].", ".$_SESSION['username'];
?>
where "lang.php" is something like:
<?php
$lang = array();
switch($_SESSION['language']) {
case "Francais":
$lang['welcome']="Bienvenue";
break;
case "Deutsch":
$lang['welcome']="Wilkommen";
break;
default:
$lang['welcome']="Welcome";
}
I'm not sure that they're coded in "One Big Switch", but you get the idea.
HTH,