[TASK] Add more descriptive information to locales class

This change updates some comments for better understanding
TYPO3-internal system languages vs. locales, and makes the
class strict.

Also the type annotations are more specific now, and an
additional type check has been introduced.

Resolves: #99108
Releases: main
Change-Id: I9ea61bed9770c9b5a13bd7cdbb1da8e889905dca
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76644


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>