[TASK] Add field `flag` to LanguageMenuProcessor

This patch adds the missing field `flag` from the site configuration to
the LanguageMenuProcessor. The field could be used now to show the flag
icon in a language menu out of the box and without workarounds.

Resolves: #84926
Releases: master, 10.4
Change-Id: Ie27bed551a96cae8c6b20913105ff95db7f61942
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65800


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Alexander Nitsche <typo3@alexandernitsche.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Alexander Nitsche <typo3@alexandernitsche.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>