Add hreflang tags to XML Sitemaps and remove them from web pages

[a-kyne]

Description

Our pages will probably load faster if we include Hreflang tags in the XML Sitemaps rather than in HTML, so that hreflangs do not need to be generated when a website URL is requested.

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Success Criteria

• [ ] Web pages do not contain hreflang tags
• [ ] A sitemap is no more than 50MB (uncompressed) and contains fewer than 50,000 URLs
• [ ] The language selection page at http://www.mozilla.org/ uses hreflang="x-default" and is grouped with the hreflang tags for the localized home pages
• [ ] Each URL in a sitemap much include hreflang tags, and each URL's hreflang tags must list itself as well as all other language versions
• [ ] Alternate URLs must be fully-qualified, including the transport method (http/https), so: https://example.com/foo, not //example.com/foo or /foo
• [ ] All language-specific URLs should be targeted at a specific country, e.g. hreflang="en-ca" or hreflang="en-gb"
• According to Google, “This can improve your rankings in the target country”
• [ ] The hreflang tags should also include one URL for each language that is also geographically unspecified, e.g. hreflang="en" or hreflang="fr" (this is new feature in our hreflangs)
• According to Google, “it's a good idea to also provide a catchall URL for geographically unspecified users of that language…It can be one of the specific pages, if you choose.”

Example:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:xhtml="http://www.w3.org/1999/xhtml">
<url>
    <loc>https://www.mozilla.org/</loc>
    <xhtml:link
               rel="alternate"
               hreflang="x-default"
               href="https://www.mozilla.org/"/>
    <xhtml:link
               rel="alternate"
               hreflang="en-ca"
               href="https://www.mozilla.org/en-CA/"/>
    <xhtml:link
               rel="alternate"
               hreflang="en-gb"
               href="https://www.mozilla.org/en-GB/"/>  
  </url>
<url>
    <loc>https://www.mozilla.org/en-US/page.html</loc>
    <xhtml:link
               rel="alternate"
               hreflang="en-us"
               href="https://www.mozilla.org/en-US/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="en-ca"
               href="https://www.mozilla.org/en-CA/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="en-gb"
               href="https://www.mozilla.org/en-GB/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="en"
               href="https://www.mozilla.org/en-US/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="de-de"
               href="https://www.mozilla.org/de/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="de"
               href="https://www.mozilla.org/de/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="fr-fr"
               href="https://www.mozilla.org/fr/page.html"/>
    <xhtml:link
               rel="alternate"
               hreflang="fr"
               href="https://www.mozilla.org/fr/page.html"/>
  </url>
</urlset>

[janbrasna]

FYI this seems to have been described in the past: (…so additional info may be there as well)

• 13892