Codes / x-default
hreflang x-default
x-default marks the URL for visitors whose language or region matches no other alternate in the cluster.
Google treats x-default as a recommendation, not a requirement.
The value belongs on language selectors, on pages redirecting by
location, and on clusters serving more than one language. The best
target is the least specific version of the page, such as the
en URL rather than en-US, or a separate
language overview page.
<link rel="alternate" hreflang="x-default" href="https://example.com/" />