[TASK] Clean up FormEngine fields for Site Handling

The descriptions and listings of site language in FormEngine
hasn't been worked on since its introduction.

This is now streamlined in terms of wording and grouping,
to help people find things in a better way.

Resolves: #86470
Releases: master
Change-Id: I3692b8466c779b32d8a52d58469e5b87874643fa
Reviewed-on: https://review.typo3.org/58489


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>