[FEATURE] Automatic language key handling with regions (ISO 3166-1)

TYPO3 can now handle language keys with a given region,
such as "de-AT" without extending TYPO3's native language
handling. When a "de-AT.locallang.xlf" file exists,
it is used, but will fall back to "de.locallang.xlf" itself.

The language is based on ISO 639-1,
whereas the region should be used from ISO 3166-1.

For backwards-compatibility, locales with underscore
are also possible "de_AT".

Resolves: #86913
Releases: main
Change-Id: I1bb97f7f36053c17bf919a92b3739e59b20d65c9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77375


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>