Add multilingual support (DE default, EN under /en/)

index.php

@@ -6,8 +6,12 @@ require_once __DIR__ . '/lib/Session.php';
 
 $session = new Session();
 
+$lang = (isset($_GET['lang']) && $_GET['lang'] === 'en') ? 'en' : 'de';
+Helper::setLang($lang);
+$session->set('lang', $lang);
+
 $loadFile = function($filename) {
-    global $config, $session;
+    global $config, $session, $lang;
 
     ob_start();
     include $filename;