TYPO3 Core

So a small update: I've written a little helper-class which reads the current session-id (accessible via cookie), a SQL query to fetch the "ses_userid"-column and then a findByUid using the BackendUserRepository. Not the best approach but it's honest work.

<?php

declare(strict_types=1);

namespace Vendor\Namespace\Helper;

use TYPO3\CMS\Beuser\Domain\Model\BackendUser;
use TYPO3\CMS\Beuser\Domain\Repository\BackendUserRepository;
use TYPO3\CMS\Core\Database\ConnectionPool;
use TYPO3\CMS\Core\Utility\GeneralUtility;

class BackendUserHelper
{
    /**
     * @var BackendUserRepository
     */
    protected static $backendUserRepository;

    /**
     * Fetch the current logged-in Backend-User by reading the cookie which
     * represents the logged-in BE-User's session-id and via queries it fetches
     * the uid of that one and additionally a findByUid using the BackendUserRepository.
     * 
     * @return BackendUser|null
     */
    public static function getUser()
    {
        $cookieSesId = $_COOKIE["be_typo_user"];

        if(!$cookieSesId) {
            return null;
        }

        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable("be_sessions");
        $res = $queryBuilder
            ->select("ses_userid")
            ->from("be_sessions")
            ->where(
                $queryBuilder->expr()->eq("ses_id", $queryBuilder->createNamedParameter($cookieSesId))
            )
        ->execute()
        ->fetch(0);

        if(!$res) {
            return null;
        }

        self::$backendUserRepository = GeneralUtility::makeInstance(BackendUserRepository::class);

        $sesUserId = $res["ses_userid"];

        $backendUser = self::$backendUserRepository->findByUid($sesUserId);
        return $backendUser;
    }
}

No, I was in the backend and tried to get the backend.user (via Context api) - primary its current language:
-> "(...) to be specific, in /Configuration/TCA/Overrides/pages.php - I guess it's due to caching of it.."

Honestly, after 7 months, I don't even think the issue is relevant at all no more.
If I remember correctly I've somehow found another way to get it within the "uc" of an user in the BE.

So can be closed, thanks though. :)