fix for having same URLSegment in many languages
3.1 fix
Resolved conflicts before merging
FIX: PrefixLink error if Translatable is not enabled
Enable the use of Duplicate URLSegments for different languages

This used in the CMS only. By replacing the Translatable extension by
the LanguagePrefixTranslatable extension late  (in updateCMSFields) all
Translatable checks on its own existance will still be validated. The
frontend has no use for this extension, because it already supports
this.