[BUGFIX] L10n fallback does not work for ExtJS in BE

PageRenderer does not take language dependency into account when reading
localization files to be included as JavaScript labels.

Change-Id: I23d12dd06a99e72968a26d5930decb87901e8af6
Fixes: #44273
Releases: 4.6, 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/17313
Reviewed-by: Mattias Nilsson
Reviewed-by: Begoña Garea Garcia
Tested-by: Begoña Garea Garcia
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers