Currently the sitemap.xml contains a seperate entry for each node, because it doesn't recognize asymmetric translated nodes as translations of one another. The sitemap should contain translation sets instead.
Example:
<url>
<loc>https://example.com/nl/webpage</loc>
<xhtml:link rel="alternate" hreflang="nl" href="https://example.com/nl/webpage"/>
<xhtml:link rel="alternate" hreflang="en" href="https://example.com/en/webpage"/>
<xhtml:link rel="alternate" hreflang="de" href="https://example.com/de/webpage"/>
<lastmod>2022-08-30T16:02:39+02:00</lastmod>
<priority>0.5</priority>
</url>
Can we hook into sitemap generation? Not sure yet.
Fixed
1.0
Code
I haven't looked into it yet, by it looks like the Sitemap module provides a way to add custom Sitemap generators (see /nl/admin/config/search/simplesitemap/types).
Automatically closed - issue fixed for 2 weeks with no activity.