[TYPO3-mvc] Translation in extension only working when logged into BE

[Urs Brauchli]

I have an extbase extension that translates some variables on pages using this

if (is_array($row) && $row['sys_language_uid'] != $GLOBALS['TSFE']->sys_language_content && $GLOBALS['TSFE']->sys_language_contentOL) {
     $row = $GLOBALS['TSFE']->sys_page->getRecordOverlay($table, $row,$GLOBALS['TSFE']->sys_language_content, $GLOBALS['TSFE']->sys_language_contentOL);
}

But there's a strange bug. It only works, when I'm logged into the backend. As soon as I log out, I can only see the default language.

Using Typo3 8.7

Any ideas what this could cause?