[BUGFIX] L10n fallback does not work for TS labels

When retrieving a label in TypoScript in a locale that is not
available locally, the English label is returned instead of
using the locale fallback mechanism introduced in TYPO3 4.6.

Change-Id: I29f4b25e3fc23497d6afe74659810ca8995a39df
Fixes: #44099
Relates: #41996
Releases: 4.6, 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/17247
Reviewed-by: Mattias Nilsson
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers