`FrontendContainerFactory` does not correctly resolve children in connected mode with `fallbackType: fallback`

[prathers]

Extension version: 3.2.2
TYPO3 version: 13.x
PHP version: 8.4

In a multilanguage setup using fallbackType: fallback in site config, content elements inside containers are not rendered in any non-default language on the frontend, even when the container and all its children are fully translated in connected mode.
Switching to fallbackType: free makes the content visible, but this is not an acceptable solution as it disables language fallback sitewide.

Root cause

FrontendContainerFactory::buildContainer() fetches children using:

$conf = ['where' => 'tx_container_parent=' . $record['uid'], ...];

Where $record['uid'] is the UID of the translated container record. This only works if translated child records have tx_container_parent pointing to the translated container UID.

However, TYPO3's connected mode translation workflow stores translated children with tx_container_parent pointing to the default language container UID — not the translated container UID. This is consistent with how l18n_parent works for standard content elements.

The backend ContainerFactory::buildContainer() handles this correctly by fetching children of the default language container and then resolving their translations via l18n_parent:

// connected mode
$defaultRecords = $this->children($defaultRecord, 0);
$childRecords = $this->localizedRecordsByDefaultRecords($defaultRecords, $language);

The frontend factory does not mirror this logic, causing it to find zero children for translated containers in connected mode.
The only partial workaround in FrontendContainerFactory is:

if ($languageAspect->getOverlayType() === LanguageAspect::OVERLAYS_OFF && $record['l18n_parent'] > 0) {
    $conf['where'] .= ' OR tx_container_parent=' . $record['l18n_parent'];
}

But this is gated on OVERLAYS_OFF which corresponds to fallbackType: free — it never runs with fallbackType: fallback.

Steps to reproduce

1. Set up a multilanguage TYPO3 13 site with a default language and one or more additional languages using fallbackType: fallback in site config
2. Create a container element in the default language with child content elements
3. Translate the container and its children in connected mode
4. View the page in a non-default language on the frontend — container children are not rendered
5. Switch fallbackType to free — content becomes visible

Content inside translated containers should be visible on the frontend in connected mode with fallbackType: fallback, consistent with how the backend ContainerFactory resolves children.

Suggested fix

FrontendContainerFactory::buildContainer() should detect connected mode (i.e. $record['l18n_parent'] > 0) and mirror the backend logic: fetch children of the default language container, then resolve their translations via l18n_parent, rather than querying by the translated container UID directly.

[prathers]

FrontendContainerFactory.php

This fix work by us