TYPO3 Core

Example Site Configuration:

rootPageId: 2
base: 'http://localhost/'
baseVariants: {  }
languages:
  -
    title: Deutsch
    enabled: true
    languageId: '0'
    base: /
    typo3Language: de
    locale: de_DE.utf-8
    iso-639-1: de
    navigationTitle: ''
    hreflang: ''
    direction: ''
    flag: de
  -
    title: English
    enabled: true
    languageId: '1'
    base: /en/
    typo3Language: default
    locale: en_US.utf-8
    iso-639-1: en
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: strict
    fallbacks: ''
    flag: gb

1. use LanguageAspectFactory::createFromSiteLanguage() to create a LanguageAspect from the config above
2. call LanguageAspect::getLegacyLanguageMode

Expected result (as it was in 9.5.5 and before):
strict

Current result (9.5.6 and 9.5.7)
content_fallback

This affects how extbase handles translated records if you disable consistentTranslationOverlayHandling (see \TYPO3\CMS\Extbase\Persistence\Generic\Storage\Typo3DbQueryParser::getSysLanguageStatement)