[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
Reviewed-on: https://review.typo3.org/18262