Is defining a sitemap spanning multisites possible?

Yes, it should be possible. Or at least in theory… But I’m not 100% sure how Multilingualpress works so please explain:
Does it create a sub-site for each language or does it use the main site for the primary language and then create sub-sites for additional languages?

I suppose it will redirect visitors depending on browser language or on user language choice but what I’m basically asking is: can you access the main site /sitemap.xml index without getting redirected to a language sub-site/dir like /de/sitemap.xml depending on your browser language, default or language preference?

Maybe if you share a link to your multisite so I can take a closer look?

O, sorry. You wrote “considering” so you do not have it set up with Multilingualpress yet… In that case, you might also consider Polylang, which has been tested with this sitemap plugin.

But if you wish to go with Multilingualpress then I’ll try to accompany you in trying to make the sitemap (index) work for all languages ??

Hi Ravan and thanks for your interest in the matter.

I only set up a local site at the moment. The primary language can be reached via the domain, English as the secondary language resides in a subdirectory /en/. I didn’t plan to redirect visitors, but I’m not through considering the pros and cons, yet.

How would you configure the settings? If I understand your description right, I could set up a distinct sitemap for each of the sites and add a link to the subdirectory’s sitemap to the domain’s sitemap?

Well, since the primary language is on / and other languages will be on /xx/ the primary language sitemap index (on /sitemap.xml) will be seen by search engines as the main sitemap index. This means you could include links to the other sitemaps (like /en/sitemap-posttype-post.xml etc) in your main index. There is an option on Settings > Reading to included external sitemaps. Add the URLs of all the other sitemaps there.

Sadly, it’s not allowed to simply reference the sub-language sitemap indexes from the main index. Instead you’ll have to add each sitemap (featuring inside each index) individually.

Or you could add the sitemaps to the robots.txt output. For that, there is also an option on Settings > Reading (on the main site only) where you can add lines like this to reference each sub-language index:

Sitemap: https://yoursite.com/en/sitemap.xml

This last one is easiest. But to monitor all sitemaps, you will have to submit each language sitemap index separately in your Google Search Console account.

Hope this helps ??

Very good, thanks a lot for filling in the details!