[BUGFIX] Trigger deprecation when using $TSFE->sys_language_*

When deprecating TSFE->sys_language_mode and TSFE->sys_language_uid
it was forgotten (by myself) to set the visibility to protected.

This way, the deprecation layer (with compat calls) is never
used, and legacy code gets wrong results.

Resolves: #86285
Related: #85543
Releases: master
Change-Id: I2d809b577e5265054f2f54d54ecfbb8e441b6bdc
Reviewed-on: https://review.typo3.org/58303


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>