To switch the language for users based on the user's browser language settings, you can do the following. Open the file includes/core.php and go to line 89 to find the code below
$_language=$config["default_lang"];
You will need to replace that by
if($lang = substr($_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 2) &&
file_exists(ROOT."/includes/languages/".Main::clean($lang,3,TRUE).".php")){
$_language = $lang;
} else {
$_language = $config["default_lang"];
}