Simple Machines Forum uses this exact same method to achieve multi-language installations of their forum. They create a massive array called $txt and fill it with every single piece of text on the site, then call it up when generating output. It would look something like this:
Code:
GB.txt.php
<?
$txt['the_color_red'] = 'red';
$txt['hello_message'] = 'hello';
?>
FRE.txt.php
<?
$txt['the_color_red'] = 'Rouge';
$txt['hello_message'] = 'Bonjour';
?>
Then, you dynamically include either txt.php file based on the user's preference, in your case, it's a $_GET variable:
Code:
index.php
<?
if(isset($_GET['lang'])) {
include($_GET['lang'] . '.txt.php');
}
else {
include('GB.txt.php');
}
echo "<body>
<div id="hello">
$txt['hello_message']
</div>
<div id="colour">
$txt['the_color_red']
</div>
You can see with this system it is very easy to add any language with any number of strings.