It’s my understanding that cross-site submissions of sitemaps are allowed by Google only if the sites are verified in the search console, don’t know how that affects a multi-lang site. (source)
I think for your use case, I would do the following:
- A single language sitemap for your default language
- Using hreflang attributes to indicate variations of the same page in all languages.
Why? Because hreflangs are used by robots when crawling to build the equivalent of a multi-lang sitemap. We built a tutorial on how to use hreflang on macotuts.com along with pre-built code. I’d be interested to hear whether the returned hreflang tags work in your setup. Feel free to start a PM chat if you want to 