[FEATURE] Make SiteLanguage available in TypoScript

[FEATURE] Make SiteLanguage available in TypoScript

The current language configuration should be available in TS as well.

Resolves: #86973
Releases: master, 9.5
Change-Id: Iae71b550b3d000b12b4e127fee751740c3a36bf7
Reviewed-on: https://review.typo3.org/58930
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>