My possible solution for now is to override NewRepository idList constraint and add l10n_parent:

$idList = $demand->getIdList();
if ($idList) {
    $commaSeparatedIdList = GeneralUtility::intExplode(',', $idList, true);
    $constraints['idList'] = $query->logicalOr(
        $query->in('uid', $commaSeparatedIdList),
        $query->in('l10n_parent', $commaSeparatedIdList)
    );
}

not so sure if this is enough as it could include other news as well

I have taken a deeper look in TYPO3's repository function "findByUid/findByIdentifier" to understand what is happening when TYPO3 fetches an object directly by it's uid, it sets three query settings:

respectStoragePage => false
Makes sense as we don't want PID restrictions when we ask for a specific uid.
I know that this setting is also set if you as an editor don't select a startpoint page but I had some issues when an editor switches plugin mode and flexform setting was hidden but processed in controller.

respectSysLanguage => false
Backend editors may select record's default language in plugin's flexform field, so don't care what language was selected

languageOverlayMode => true
Overlay selected record to fit current website language