Are you asking about sitemaps for humans or sitemaps for search engines?
Search engines don't give a ### what language your page is in. If it exists, they want to index it.* Language is just one more detail. What is important is to make sure each individual page carries the appropriate language information. That's the only way a search engine can get it right when people ask for "only pages written in language nnn". Unlike a human, a search engine can't just look with its eyeballs and see that this page is in French.
If the pages are linked from anywhere, the search engine will find them. The sitemap is just insurance.
* Except Yandex, which-- unlike g###-- is honest about which languages it doesn't know.