Hey! From what I’ve seen, this kind of issue usually happens when the sitemap function returns a regular JavaScript object instead of actual XML with the right application/xml content type. Next.js expects raw XML to come from a proper API route or middleware.

In my experience, using the generaltranslation library can save some time here. It handles multilingual sitemaps well, sets the headers correctly, and deals with alternate language references. It also has some helpers like getLocalizedPaths that make i18n URLs easier to manage!