Feature: #107436 - Symfony Translation Component integration 

Description 

TYPO3 now utilizes the Symfony Translation component for reading localization label files such as XLIFF and PO, instead of its custom localization parsers.

The migration brings several improvements:

• Standardized file parsing using Symfony's translation loaders
• Enhanced API for accessing translation catalogues
• Support for custom translation loaders following Symfony standards

The new system maintains backward compatibility while providing a modern foundation for future improvements with translatable labels.

In addition, all label-related configuration options have been streamlined under the $GLOBALS['TYPO3_CONF_VARS']['LANG'] namespace.

The following new configuration options have been introduced:

$GLOBALS['TYPO3_CONF_VARS']['LANG']['loader']

Configure custom translation loaders.

$GLOBALS['TYPO3_CONF_VARS']['LANG']['requireApprovedLocalizations']

Moved from SYS.lang.

$GLOBALS['TYPO3_CONF_VARS']['LANG']['format']

Moved from SYS.lang.

$GLOBALS['TYPO3_CONF_VARS']['LANG']['availableLocales']

Moved from EXTCONF.lang.

$GLOBALS['TYPO3_CONF_VARS']['LANG']['resourceOverrides']

Moved from SYS.locallangXMLOverride.

Custom translation loaders 

Extension developers can now implement custom translation loaders by implementing Symfony's translation loader interfaces:

use Symfony\Component\Translation\Loader\LoaderInterface;
use Symfony\Component\Translation\MessageCatalogue;

class CustomLoader implements LoaderInterface
{
    public function load(mixed $resource, string $locale, string $domain = 'messages'): MessageCatalogue
    {
        // Custom loading logic
        $catalogue = new MessageCatalogue($locale);
        // ... populate catalogue
        return $catalogue;
    }
}

Register custom loaders via configuration:

$GLOBALS['TYPO3_CONF_VARS']['LANG']['loader']['fileEnding']
    = \MyVendor\MyExtension\Translation\CustomLoader::class;

Impact 

All previous configuration options have been moved to the new $GLOBALS['TYPO3_CONF_VARS']['LANG'] namespace. These are automatically migrated to the new location when accessing the install tool.

Please note: This functionality only affects the internal handling of translation files ("locallang" files). The public API of the localization system remains unchanged.