[TASK] Simplify Locales class

The Locales class implies a kind-of singleton approach to fetch
configuration information.

In addition, Locales does the same functionality all over again:
- array_flip for "isoReverseMapping" (not in use directly and can be removed)
- taking the built-in supported languages and loops over them

both is unnecessary at runtime.

Instead, only the user-specific parts are validated.

Resolves: #88488
Releases: master
Change-Id: Ib1a9589bd670fe77b5bf7f5ca769c9601a2b2b0c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60877


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>