I'm curious what tweaks are being done to the generated sitemaps?

I'm curious what tweaks are being done to the generated sitemaps?

some logic regarding languages, we want rewrite some hrefs in xml

The reason I'm asking is to determine whether or not there is something in the way SEOmatic generates the sitemaps that needs to be corrected, or whether this is a one-off special case where you need to rewrite the output?

Well, in craft we have some sites in english, but for one of them we want to change the hreflang 'en' -> 'en-gb', but we don't want to change the site language, it should stay 'en'. Haven't found in SEOmatic any special config matching site language and sitemap language, so we thought we could use event hook for this.

UPD. So I think it's a kind of special case and a bit hacky.

Are you also then changing the hreflang meta tags manually on the corresponding pages as well?

Are you also then changing the hreflang meta tags manually on the corresponding pages as well?

yes