[BUGFIX] Fix handling menuConfig of LanguageMenuProcessor

Use array_merge instead of union (+=) in order to
correctly handle overriding the menu configuration
of the LanguageMenuProcessor.

Resolves: #88123
Releases: master, 9.5
Change-Id: I1f382c11b2f24c052b97af2b7d62a6e0712e10e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60432


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>