[BUGFIX] LanguageController has "mixed" type annotations

The LanguageController of the lang sysext throws
exceptions if the rewritten property mapper is enabled.
The type annotation 'mixed' is unknown. The patch fixed
the annotation to the correct string value to make the
property mapper happy.

Change-Id: I57ebd268d2fe82c095ccf20b581e721703fb2868
Fixes: #44672
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/18920
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn