Generate and validate hreflang annotations in XML sitemaps for international sites, including x-default and reciprocal link requirements

Steps

1. Add the sitemap namespace 'xmlns:xhtml="http://www.w3.org/1999/xhtml"' to the root <urlset> element
2. For each URL, include one <xhtml:link rel="alternate" hreflang="{lang-code}" href="{url}"/> tag per language/region variant inside the <url> block, plus one pointing back to itself
3. Add an x-default variant using hreflang="x-default" pointing to the URL that should be shown when no other language/region matches the user — typically a language selector or the global homepage
4. Ensure full reciprocity: every URL referenced in a hreflang annotation must itself contain a hreflang set pointing back to all variants including the page that references it
5. Validate language codes using ISO 639-1 for language (e.g., 'en', 'fr') and optionally ISO 3166-1 alpha-2 for region (e.g., 'en-US', 'fr-FR'); do not use BCP 47 variants not supported by Google
6. Crawl and validate the sitemap using a tool such as Screaming Frog or hreflang.org validator to detect missing return links, incorrect codes, and canonical conflicts before submission