pageNotFound_handling for restricted pages in TYPO3

AdditionalConfiguration.php

$GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling'] = 'USER_FUNCTION:EXT:usermytemplate/Classes/Utility/PageNotFoundHandling.php:user_pageNotFound->pageNotFound';

// Custom configuration for multi-language 404 page, see EXT:usermytemplate/Classes/Utility/PageNotFoundHandling.php
// ID of the page to redirect to if page was not found
$GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling_redirectPageID'] = 123;
// ID of the page to redirect to if current page is access protected
$GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling_loginPageID'] = 789;

PageNotFoundHandling.php

<?php

use TYPO3\CMS\Core\Utility\GeneralUtility;

class user_pageNotFound {
   /**
    * Detect language and redirect to 404 error page
    *
    * @param array $params "currentUrl", "reasonText" and "pageAccessFailureReasons"
    * @param \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController $tsfeObj
    */
   public function pageNotFound($params, $tsfeObj) {

    
     // get the language first
     // get first realurl configuration array (important for multidomain)
     $realurlConf = array_shift($GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['realurl']);
     // look for language configuration
     foreach ($realurlConf['preVars'] as $conf) {
       if ($conf['GETvar'] == 'L') {
         foreach ($conf['valueMap'] as $k => $v) {
           // if the key is empty (e.g. default language without prefix), break
           if (empty($k)) {
             continue;
           }
           // we expect a part like "/de/" in requested url
           if (GeneralUtility::isFirstPartOfStr($params['currentUrl'], '/' . $k . '/')) {
             $langUid = $v;
           }
         }
       }
     }


      /*
       * If a non-existing page with a RealURL path was requested (www.mydomain.tld/foobar), a fe_group value for an empty
       * key is set:
       * $params['pageAccessFailureReasons']['fe_group'][null] = 0;
       * This is the reason why the second check was implemented.
       */
      if (!empty($params['pageAccessFailureReasons']['fe_group']) && !array_key_exists(null, $params['pageAccessFailureReasons']['fe_group'])) {
         // page access failed because of missing permissions
         header('HTTP/1.0 403 Forbidden');
         $this->initTSFE(1);
         /** @var \TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer $cObj */
         $cObj = GeneralUtility::makeInstance('TYPO3\\CMS\\Frontend\\ContentObject\\ContentObjectRenderer');
         $loginUrl = $cObj->typoLink_URL(array(
            'parameter' => $GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling_loginPageID'],
            'useCacheHash' => FALSE,
            'forceAbsoluteUrl' => TRUE,
            'additionalParams' => '&redirect_url=' . $params['currentUrl'] . '&L=' . $langUid
         ));
         TYPO3\CMS\Core\Utility\HttpUtility::redirect($loginUrl);
      } else {
         // page not found
      $tsfeObj->pageErrorHandler('/index.php?id=' . $GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling_redirectPageID'] . '&L=' . $langUid);
         // handle default language
         $tsfeObj->pageErrorHandler('/index.php?id=' . $GLOBALS['TYPO3_CONF_VARS']['FE']['pageNotFound_handling_redirectPageID']);
      }
   }

   /**
    * Initializes a TypoScript Frontend necessary for using TypoScript and TypoLink functions
    *
    * @param int $id
    * @param int $typeNum
    */
   protected function initTSFE($id = 1, $typeNum = 0) {
      \TYPO3\CMS\Frontend\Utility\EidUtility::initTCA();
      if (!is_object($GLOBALS['TT'])) {
         $GLOBALS['TT'] = new \TYPO3\CMS\Core\TimeTracker\NullTimeTracker;
         $GLOBALS['TT']->start();
      }

      $GLOBALS['TSFE'] = GeneralUtility::makeInstance('TYPO3\\CMS\\Frontend\\Controller\\TypoScriptFrontendController',  $GLOBALS['TYPO3_CONF_VARS'], $id, $typeNum);
      $GLOBALS['TSFE']->sys_page = GeneralUtility::makeInstance('TYPO3\\CMS\\Frontend\\Page\\PageRepository');
      $GLOBALS['TSFE']->sys_page->init(TRUE);
      $GLOBALS['TSFE']->connectToDB();
      $GLOBALS['TSFE']->initFEuser();
      $GLOBALS['TSFE']->determineId();
      $GLOBALS['TSFE']->initTemplate();
      $GLOBALS['TSFE']->rootLine = $GLOBALS['TSFE']->sys_page->getRootLine($id, '');
      $GLOBALS['TSFE']->getConfigArray();

      if (\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::isLoaded('realurl')) {
         $rootline = \TYPO3\CMS\Backend\Utility\BackendUtility::BEgetRootLine($id);
         $host = \TYPO3\CMS\Backend\Utility\BackendUtility::firstDomainRecord($rootline);
         $_SERVER['HTTP_HOST'] = $host;
      }
   }

}

determine $langUid first and then also use for pageAccessFailureReasons