[BUGFIX] Allow "en" as language key

Fix a bug which was introduced by #25397 (commit:cb390c79).
If the selected language key is "en", do not override it to "default".
This allows custom updates to translations even for the English language.

Change-Id: I3fcbd35f0144a0cc152acd45d0861d153cc8b495
Resolves: #42084
Related: #25397
Releases: 4.6, 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/17414
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
[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
[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
Merge remote-tracking branch 'upstream/master'