[TASK] Tell reason why translation wizard is not usable in "mixed mode"

When clicking the "translate" button for a page translation, that has
"mixed mode" records, an empty wizard would be shown with no indication
what is wrong.
An alert message is now added, that explains the reason why the
translation wizard cannot be used in "free mode".

Resolves: #89345
Releases: master
Change-Id: I81afe42330393e9c2135e987e838a674b4f5288b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62788


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Steffen Frese <steffenf14@gmail.com>
Tested-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Steffen Frese <steffenf14@gmail.com>
Reviewed-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>