Earlier quoted context omitted.
I don't know what it uses to output non-English text, but if you address it in a different language, it'll quite happily speak that back to you. And I'd say it understands it just fine, not really any worse than English - although it sometimes gets the word forms wrong in inflective languages.
But I was asking it for rhyming words in Dutch (because of Sinterklaas), and it suggested words that would rhyme if pronounced as English...
I think that makes sense if the supermajority of the training data was in English - it'd make it heavily biased towards that by default. So even when it speaks another language, it still "thinks" of the words in terms of questions like "how it sounds". I suspect it might be possible to craft a sufficiently detailed prompt to work around that.