Как правильно реализовать мультиязычность?

<?php
session_start();
//print_r($_SERVER["HTTP_ACCEPT_LANGUAGE"]);
//unset($_SESSION);
//echo "session: " . print_r($_SESSION, true) . "\n";
$accepted_languages = explode(",", $_SERVER["HTTP_ACCEPT_LANGUAGE"]);
//print_r($accepted_languages);
if (isset($_GET["lang"])) $_SESSION["lang"] = $_GET["lang"];
// give session when first load page - дать сеанс при первой загрузке страницы
if(!isset($_SESSION['lang'])){
    foreach($accepted_languages as $lang_def) {
        if (strpos(" ".$lang_def, "it")) { $_SESSION["lang"] = 1; $lang = "it"; break; }
        elseif (strpos(" ".$lang_def, "en")) { $_SESSION["lang"] = 3; $lang = "en"; break; }
        elseif (strpos(" ".$lang_def, "ru")) { $_SESSION["lang"] = 2; $lang = "ru"; break; }
    }
    if(!isset($_SESSION['lang'])){ $_SESSION['lang']=3; $lang = "en"; }
}
//echo "lang: $lang\n";
 
if($_SESSION['lang']==1){
  include('lang_it.php');
}else if ($_SESSION['lang']==2) {
  include('lang_ru.php');
}if ($_SESSION['lang']==3) {
  include('lang_en.php');
}
 
if ($_SESSION['lang'] == "2") $lang = "ru";
if ($_SESSION['lang'] == "1") $lang = "it";
if ($_SESSION['lang'] == "3") $lang = "en";
?>

<?php
session_start();
 
// получить текущий URL
$goback = $_SERVER['HTTP_REFERER'];
$lang=$_GET['lang'];
$_SESSION['lang']=$lang;
//echo $goback;
//переход к текущему URL
header("Location: $goback");
 ?>

<?php
//ini_set('display_errors',1);
//error_reporting(E_ALL);
 
$select = mysql_query("SELECT * FROM tblang ");
$numrow = mysql_num_rows($select);
if ($numrow > 0) {
 
while ($row = mysql_fetch_object($select)){
 
if ($_SESSION['lang']==1) {
$how = $row->it;
$how_id = $row->id;
$it[$row->id] = $row->it;
}else
 
if ($_SESSION['lang']==2) {
 
$how = $row->ru;
$how_id = $row->id;
$ru[$row->id] = $row->ru;
}  else
if ($_SESSION['lang']==3) {
$how = $row->en;
$how_id = $row->id;
$en[$row->id] = $row->en;
}
//echo $how.'<br>';
}
}
?>