"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."