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Hello @mswebadm,
Thank you for contacting the support, and sorry for the inconvenience caused.
Unfortunately, Rank Math is still not yet 100% compatible with Polylang, which can be why the category URLs of secondary languages are not included in the sitemap.
We have been trying to collaborate with them for a few years but left the conversation when they refused to add the compatibility even though they have it for several other plugins (some of them have a smaller user base compared to RM)
https://github.com/polylang/polylang/tree/master/integrations
You can check their public repository, where we created a pull request (linked to from your opening post) a few years back, and we would have maintained it (though that would not have happened ideally since we do not make changes in the plugin that breaks the compatibility).
Disappointingly, compatibility needs to be added in their plugin as they added for other SEO plugins. We are more than committed to making that happen and always keep it updated if their team agrees and wants to collaborate.
However, even if the URLs are not part of the sitemap, the category URLs of the main language should have the hreflang tags for other languages, and Google will be able to find the secondary language URLs by following the hreflang tags.
Hope that helps. Let us know if you need any other assistance.