TYPO3 Core

OK, we have a quite complicated setup... but its working and its necessarry, because parts of the website have different default languages in BE.

Here is a snippet from the a language-configuration in a site:

...
languages:
  -
    title: Deutsch
    enabled: false
    languageId: '0'
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: Deutsch
    hreflang: de-DE
    direction: ltr
    flag: de
  -
    title: 'Übersetzung Deutsch (nicht verwendet)'
    enabled: true
    languageId: '2'
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: Deutsch
    hreflang: de-DE
    direction: ltr
    fallbackType: free
    fallbacks: '0'
    flag: de
...

So effectively in FE the languageUid 2 is used but falls back to 0 (which is used in the BE)
This works great, BUT previewing hidden pages is not possible, because the preview does not respect the fallback defined in the site language.