[FEATURE] Allow user-defined additional backend languages

[FEATURE] Allow user-defined additional backend languages

At the moment, any language may be used for Frontend output. But in Backend, the list
of supported languages is hardcoded at various places. This prevents additional
languages to be defined locally, for instance when translating TYPO3 into a new
language, before it gets officially supported by the Core.

Change-Id: I66cd9149c860746356e249c778a5303211076d37
Resolves: #29513
Releases: 4.6
Reviewed-on: http://review.typo3.org/4830
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Dominique Feyer
Tested-by: Dominique Feyer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers