Show translated labels in selectbox of relations in backend

TcaSelectItemsProcessor.php

<?php

namespace Elo\EloCareer\Tca;

/*
 * This file is part of the TYPO3 CMS project.
 *
 * It is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License, either version 2
 * of the License, or any later version.
 *
 * For the full copyright and license information, please read the
 * LICENSE.txt file that was distributed with this source code.
 *
 * The TYPO3 project - inspiring people to share!
 */

use TYPO3\CMS\Backend\Form\FormDataProvider\TcaSelectItems;
use TYPO3\CMS\Backend\Utility\BackendUtility;
use TYPO3\CMS\Core\Database\ConnectionPool;
use TYPO3\CMS\Core\Database\Query\Restriction\DeletedRestriction;
use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * Class TcaSelectItemsProcessor
 */
class TcaSelectItemsProcessor
{
    /**
     * @param array $parentArray
     * @param TcaSelectItems $parentObj
     */
    public function translateTable1Selector($parentArray, TcaSelectItems $parentObj)
    {
        $parentArray['items'] = $this->translateItemsForTable(
            $parentArray['items'],
            'tx_mytable_domain_model_table1',
            'type'
        );
    }

    /**
     * @param array $items
     * @param string $table
     * @param string $fieldName
     * @return array
     */
    protected function translateItemsForTable($items, $table, $fieldName)
    {
        if (is_array($items)) {
            foreach ($items as $key => $item) {
                $translatedRecord = BackendUtility::getRecordLocalization(
                    $table,
                    (int)$item[1],
                    $this->getLanguageUidFromBeUser()
                );
                if (!empty($translatedRecord)) {
                    $items[$key][0] = $translatedRecord[0][$fieldName];
                }
            }
        }
        return $items;
    }

    /**
     * @return int
     */
    protected function getLanguageUidFromBeUser()
    {
        $userConfiguration = $this->getBackendUserAuthentication()->uc;
        $iso2 = $userConfiguration['lang'] ?: 'en';
        $queryBuilder = $this->getConnectionPool()->getQueryBuilderForTable('sys_language');
        $queryBuilder->getRestrictions()->removeAll();
        $queryBuilder->getRestrictions()->add(
            GeneralUtility::makeInstance(DeletedRestriction::class)
        );
        $row = $queryBuilder
            ->select('uid')
            ->from('sys_language')
            ->where(
                $queryBuilder->expr()->eq(
                    'language_isocode',
                    $queryBuilder->createNamedParameter($iso2, \PDO::PARAM_STR)
                )
            )
            ->execute()
            ->fetch();

        if (empty($row)) {
            return 0; // default language (german)
        }
        return (int)$row['uid'];
    }

    /**
     * @return mixed|\TYPO3\CMS\Core\Authentication\BackendUserAuthentication
     */
    protected function getBackendUserAuthentication()
    {
        return $GLOBALS['BE_USER'];
    }

    /**
     * Get TYPO3s Connection Pool
     *
     * @return ConnectionPool
     */
    protected function getConnectionPool()
    {
        return GeneralUtility::makeInstance(ConnectionPool::class);
    }
}