I understand from reading the page you linked that we should probably use the two-letter language codes instead of the language-region codes, so hreflang="de" instead of hreflang="de-DE" etc.
This does currently not work on the most important page of the site: /draft/<code>
For all other pages, it only links to the main page, not the specific sub page in other languages
Add
<link hreflang>
tags to the page using React Helmet More info https://developers.google.com/search/docs/advanced/crawling/localized-versions