Behaviour canonical link and HTML lang tag in language overlay
If you on a non-translated alternative language page and the fallback language page is shown, the HTML lang tag provides the wrong language code. It provides the code of the chosen language but it what is actually should provide is the code of the overlay language. For example if you are on a non-translated EN page the fallback language DE is shown. In this case the HTML lang tag should be DE but and not EN.
Second, the canonical link points to the path of the EN page like /en/blablabla which delivers the same content like /de/blablabla which is duplicated content. To solve this problem the canonical link of the non-translated page should point to the URL of the fallback/original page.
TYPO3 version: 9.5.7