I translate Latin, German, and Dutch to English professionally. I have enough experience to know that AI translations are educated guesses that are normally within an acceptable range of accuracy. But it hallucinates in translations to an astonishing degree. If you do not know the language you are translating from, it can fool you easily, especially with the confidence it seems to evince when answering your queries. This is the same in OpenAI, Gemini, and Grok.
I've never used Duolingo. I do not know if it is interactive or not.
If it is not interactive, I don't know why you would use it except to collect badges.
And if it is interactive, you'll be "learning" a language taught by a model that guesses based on statistics and that at times will justify its output with logical tall tales that in no way reflect the language as it is used by native speakers.