if(!empty($_REQUEST['lang'])){
$_SESSION['lang']=$_REQUEST['lang'];
}
if (!empty($_SESSION['lang']))
{
switch($_SESSION['lang']){
  case 'zh_cn':
   $_CFG['lang']="zh_cn";
  break;
  case 'zh_tw':
   $_CFG['lang']="zh_tw";
  break;
  case 'en_us':
   $_CFG['lang']="en_us";
  break;
}

}
require(ROOT_PATH . 'languages/' . $_CFG['lang'] . '/common.php');
$smarty->assign('lang', $_LANG);

删除原
require(ROOT_PATH . 'languages/' . $_CFG['lang'] . '/common.php');
$smarty->assign('lang', $_LANG);

页面调用
<a href="?lang=zh_cn">简体</a>
<a href="?lang=zh_tw">繁体</a>
<a href="?lang=en_us">英文</a>