TYPO3 Core

If a language is defined as fallback, neither that language's page nor the fallback language's page will include a hreflang to each other.

Simple language fallback configuration:

languages:
  -
    title: deutsch
    enabled: true
    languageId: '0'
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: DE
    hreflang: de-DE
    direction: ''
    flag: de
  -
    title: AT
    enabled: true
    languageId: '1'
    base: '/at'
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: AT
    hreflang: de-AT
    direction: ''
    fallbackType: fallback
    fallbacks: '0'
    flag: at

If /mypage is not translated, it is still accessible as /at/mypage, but neither /mypage nor /at/mypage will have any hreflang.