"Duo is quite smart."
Translation:Duo ist ziemlich schlau.
In English you would rarely (never?) say "He is smart" to mean "He is nicely dressed." You definitely could say "He dresses smartly" or "He is smartly dressed," and given specific context, "He looks smart," but the last example sounds particularly old-fashioned or even coy. I'd be as likely to use "smart" in that way as I would be to say, "You look groovy."
the correction here is probably not the best solution ... smart is not necessarily schlau.... schlau auf Deutsch kann auch eine negative Bedeutung haben... In this example I do not think Duo should be sly. rather Duo should be intelligent or clever...suggestion: duo ist sehr gescheit, Duo ist ganz intelligent...