[FEATURE] Provide SiteLanguageProcessor

The SiteLanguageProcessor retrieves the current siteLanguage entity as
it can be useful to have this information available in the templates.

Resolves: #96800
Releases: main
Change-Id: Idf59dfb6f644f116ece710e5a296c8e8524a21f5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73370


Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>