[TASK] Prepare Site handling for extended page routing

An additional trait for site languages and some minor simplifications
when fetching the PageRouter from a Site.

Resolves: #86410
Releases: master
Change-Id: I88fd8fccbd19aada7014e7dca04c964e87fdd0cd
Reviewed-on: https://review.typo3.org/58419


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>