function getPreferredLanguage() {
	$langs = array();
	if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
		// break up string into pieces (languages and q factors)
		preg_match_all('/([a-z]{1,8}(-[a-z]{1,8})?)\s*(;\s*q\s*=\s*(1|0\.[0-9]+))?/i',
				$_SERVER['HTTP_ACCEPT_LANGUAGE'], $lang_parse);
		if (count($lang_parse[1])) {
			// create a list like "en" => 0.8
			$langs = array_combine($lang_parse[1], $lang_parse[4]);
			// set default to 1 for any without q factor
			foreach ($langs as $lang => $val) {
				if ($val === '') $langs[$lang] = 1;
			}
			// sort list based on value	
			arsort($langs, SORT_NUMERIC);
		}
	}
	//extract most important (first)
	foreach ($langs as $lang => $val) { break; }
	//if complex language simplify it
	if (stristr($lang,"-")) {$tmp = explode("-",$lang); $lang = $tmp[0]; }
	return $lang;
}

http://www.barattalo.it/2009/12/16/detect-user-language-with-php/

Function to detect user language in php.

Detect user language with PHP

php

PHP