Implement hreflang x-default to handle users whose locale matches none of your targeted language variants

Steps

1. Identify the URL that should receive traffic from users in unspecified locales, typically a language-selector page or your default language homepage
2. Add a hreflang link element for each language-region variant of the page, plus a separate element with hreflang=x-default pointing to the fallback URL
3. Ensure every page in the hreflang set references all other pages in the set (including the x-default URL) to form a complete reciprocal annotation cluster
4. Validate reciprocity by crawling each annotated URL and confirming that each hreflang element in the cluster points back to every other URL in the same cluster
5. Choose implementation method (HTML link elements, XML sitemap xhtml:link, or HTTP Link header) based on your CMS capability; all three are treated equally by Google