"Are you smart? Yes. We are smart."
Translation:Suval'a'? HISlaH. maval.
That is correct. If you have not even been reading the Tips & Notes, I applaud you for picking up on it. Or perhaps you are just trying to confirm what you read in the Tips & Notes. Since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. Of course, we are always happy to answer question here, as well.
Based on historical versions of English, singular should really have its own separate word. "You" is the plural. "Thou" used to be the singular. As in many languages, the plural was used as a formal way of speaking even to a singular person. For some reason it became the thing to address everyone with the formal/plural "you" and people eventually just stopped using "thou" for the singular.
Because the response sentence uses "we", it should be clear that the English sentence is using a plural "you" and so the Klingon translation must use the plural version of "you" in the translation. A true dialog option would be more clear, but there are a number of exercises in this course that relies on this kind of "dialog" to make the singular/plural distinction clear. And when a user is given the Klingon sentence (which is clearly plural), it accepts English translations which use either "you" or "you all".