We have this setup for our website:
https://www.example.com/something.php→ English version
https://www.example.com/es/something.php→ Spanish version
Same for other languages.
We also have some pages that we didn't translate (yet). For example our blog, but also pages with low traffic.
If you visit
https://www.example.com/es/blog.php, you're still on the Spanish version of our website (navigation, title, … is in Spanish), but most of the content is English, and on the top there's a dialogue which say: "This page isn't translated yet to Spanish. Therefore, we show you an English version of the page", but it says so in Spanish, of course.
We added these non-translated pages to our sitemap just like we do with our translated pages. We assume however that Google will see these as duplicate content. We don't mind that they don't feature these particular pages in search, but we don't want to receive penalties resulting in poor page ranking in general of course because of this practice.
Should we worry about this? An alternative would be that we link to the English versions (
https://www.example.com/blog.php) on the Spanish pages, but then the navigation etc. changes to English too and the
/es/ part is removed from the URL, which is not ideal from a user perspective.