[BUGFIX] Use `modTSconfig` for default language label, if set

Due to a wrong assignment, modSharedTSconfig is always used
to set the default language label in PageLayoutController, even if
modTSconfig is set.

modTSconfig is now used if possible.

Resolves: #75242
Releases: master, 7.6, 6.2
Change-Id: I517c03f02ffc8d05ed74a865517ee775e1542bfe
Reviewed-on: https://review.typo3.org/47375


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>