Back in 2023, I have looked at how to add relevant hreflang tags to my personal website, which considered adding self-reference to the current language, reference to a translated page (if available), and falling back with x-default to a page that is designed as a language switcher page.
Since then, I thought that this approach was correct, but found out that this solution, on my website, where not all content is 100% translated, is not good for sites fully translated by default.
Here is how I learned from it and how I optimised hreflang and x-default on websites built with Hugo.
Setting a hreflang meta tag on your multilingual website shall be as simple as a piece of cake. Just put the relevant meta tag on your website, refer to the translated version and on translated version refer back to the original one.
Looks as simple as that. End of story? Wrong!