TYPO3 Core

Looking further into it it seems that maybe multiple file name resolvings need to work?

typo3Language: de_DE
files: de_DE.locallang.xlf -> or if missing: de-DE.locallang.xlf -> or if missing: de.locallang.xlf?

typo3Language: de
files: de.locallang.xlf

This is a deep dive through many init(...) and languageService factory plus instantiation layers.

Shortcircuiting `SiteLanguage.php::getTypoe3Language()` to this:

    public function getTypo3Language(): string
    {
        if ($this->typo3Language !== '') {
            return $this->typo3Language;
        }
        // locale is just set to "C" or "en", this should then be mapped to "default" 
        if ($this->locale->getLanguageCode() === 'en' && !$this->locale->getCountryCode()) {
            return 'default';
        }
        return $this->locale->getLanguageCode();
    }

works for my cases with "de.locallang.xlf" labels, but I'm sure this is not the way it was meant to be.

I tested this purely with a TYPO3 v13/main installation and the "fe-login" extension. No custom typoscript, extensions or anythin else. Downloading a german languagepack only creates "de.locallang.xlf" files, and not "de_DE.locallang.xlf" or "de-DE.locallang.xlf".

Patch set 2 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/82061

Patch set 1 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/82065

Patch set 2 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/82065

Patch set 1 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/82019