ChatGPT itself can speak in as many languages as there are on the internet, since it's trained on that data. It's quality is likely proportional to the amount that any language is used online on any indexable site.
From what I've used so far in other languages, I'm very impressed. It's able to understand and speak slang, a mix of other languages and English (e.g. Spanglish, Japlish, Hinglish), language written in latin text where the original language is not (romaji, romanized hindi, arabizi), and more.
I found that (with pylint as my metric) code requests in Russian, German and, strangely enough, best of all, Bulgarian, are higher quality than requests made in english (deepl for translation engine).
I still need to grep through the other data I saved from codex but I made it LARP as a distinguished professor of computer science who was unable to speak english
