Symfony version(s) affected: v5.3.7, v4.4.30

Description

With the change in #42576 the catalog fallback language gives now en_001 instead of the wanted fallback language

How to reproduce
Insert into src/Symfony/Component/Translation/Tests/TranslatorTest.php

public function testCatalogueGetsCorrectFallback()
{
        $translator = new Translator('en_GB');
        $translator->setFallbackLocales(['en_GB', 'en']);
        $catalogue = $translator->getCatalogue('en_GB');

        $this->assertEquals('en', $catalogue->getFallbackCatalogue()->getLocale());
}

Possible Solution

\Symfony\Component\Translation\Translator::computeFallbackLocales computes the fallback with the language en_GB. This has an entry in parents.json and the $parent will be en_001 and later assigned in the if as locale.

With the change of #42576, it will be appended to the $locales array and the fallback language will be then en_001 instead of en_GB

Additional context