I tried using it in Portuguese and it's cool but it seems very off, just a huge number of things that don't sound even remotely natural or idiomatic. But when I click the translate button, the English translation is a perfectly reasonable thing to say.

SO... I'm highly suspecting that, behind the scenes, the "real" conversation is happening in English, and it's translating in both directions. Which is always going to be fairly disastrous, particularly with short snippets of conversational text. (In contrast, machine translation does best with long sentences of highly technical writing.)

So the concept is very intriguing, but the problem is that it needs to rely on native language models per-language. (Or if it already is, then those models need a vastly better or larger training corpus.)

So A+ for proof of concept, but I'd never recommend anybody practice with this in order to learn/practice a language, because with the quality where it is, it will make you a worse speaker -- not better. The things it says are just too weird.