[BUGFIX] Make sure that language key is set in extbase.

If the TYPO3 context is BE and no user is logged (typically
in the login page), the language key is not set from the
browser's preferred language.

Resolves: #71097
Releases: master
Change-Id: I2521deb46083fccc2bff966b8b264a987689ee6a
Reviewed-on: https://review.typo3.org/44348


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Jérémie Roulin <jeremie@hemmer.ch>
Tested-by: Jérémie Roulin <jeremie@hemmer.ch>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>