This snippet detects the browser language and provides $available_languages as an array('en', 'fr', 'es').

detect_browser_language.php

<?php
// Source : http://snipplr.com/view/12631/detect-browser-language/

function get_client_language($available_languages, $default='en'){
	if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
		$langs=explode(',',$_SERVER['HTTP_ACCEPT_LANGUAGE']);

		foreach ($langs as $value){
			$choice=substr($value,0,2);
			if(in_array($choice, $available_languages)){
				return $choice;
			}
		}
	} 
	return $default;
}
?>