I know I would have gotten marked wrong in my French class many years ago if I had translated this a "Peux-tu parler l'italien", but that was a long time ago.
All the "able to" verbs in French, Spanish and Italian seem particularly unsuited to translating "Are you able to speak French/Spanish/Italian", because that means something like "Can you physically shape your mouth and utter the correct sounds that will be heard as French/Spanish/Italian?" It has nothing to do with knowledge of any language, simply the ability to do something - the capacity to do so.
But that was many decades ago. I've seen so many new words in the French module, I'm shocked that the Academy let them into the language.
Yes that's why I was surprised to see my answer rejected (or maybe I didn't notice that I did a mistake). There is a slight difference between "Can you..." and "Do you know how to..." indeed. However I do remember my English teachers telling me that to question someone about their ability to do something, the "Can you..." form would be the best one to use. What do you think about it ?
I think that, indeed, the "Can you..." is used more often, probably because it is shorter to say. XD But the meanings are very similar; if you know how to do something, you (hopefully) can do it, but that is not always the case. For example, I know how to fill air into bike tires and ride a bike, but, since I have not done it in so long, it took me a while to be able to do it yesterday. :P
If I understand a very informative comment by the Great CivisRomanus correctly, "Sai parlare tedesco" is colloquial Italian, whereas the more grammatically correct version would be: "Sai parlare il tedesco". On the use of the definite article with the names of languages, see CivisRomanus's entire comment: https://forum.duolingo.com/comment/22770982/l-italiano-vs-italiano