require('_inc_lang/lan_en.php');
require('_inc_lang/lan_al.php');

$lan = isset($_GET['lan']) ? $_GET['lan'] : 'AL';
switch ($lan) {
    case 'AL':
        $TEXT = $TEXT_AL;
    break;
    case 'EN':
        $TEXT = $TEXT_EN;
    break;
}

session_start();
if($_GET['lan'])
$_SESSION['lan'] = $_GET['lan'];
$lan = $_SESSION['lan'] ? $_SESSION['lan'] : 'AL';
switch ($lan) {
case 'AL':
$TEXT = $TEXT_AL;
break;
case 'EN':
$TEXT = $TEXT_EN;
break;
}

<?php

if(!isset($_SESSION['lan'])){
session_start();
}

if(isset($_GET['lan'])){
$_SESSION['lan'] = $_GET['lan'];
}

$lan = isset($_SESSION['lan']) ? $_SESSION['lan'] : 'AL';
switch ($lan) {
case 'AL':
echo 'AL';
break;
case 'EN':
echo 'EN';
break;
}

$lang = $_GET['lan'];
switch ($lang) {
    default: // use the default function of the switch instead of the isset()
    case 'AL':
        $TEXT = $TEXT_AL;
        $lang = 'AL'; // Set it AL, in case it had no value, or no matching cases
    break;
    case 'EN':
        $TEXT = $TEXT_EN;
    break;
}
$_SESSION['lang'] = $lang; // Save the sessionvariable AFTER the case.