Hreflang
The hreflang attribute declares the language or regional versions of a given page, so each user receives the correct language in their results. Every version must reference all the others, including itself, for the annotation set to be valid.
In practice
The classic failure mode: non-reciprocal annotations that invalidate the entire group. At the scale of 21 languages, markup is generated and verified automatically — manual management would not survive it. The x-default value designates the version served outside the listed languages, typically an international landing page.