Is Duolingo a joke in U.S.? I was talking to an american on Tandem I asked him where did he learned Portuguese and if it was on Duolingo, the guy sounded so offended like I offended the eighth generation of his family.
Duolingo is generally considered not the greatest tool for learning languages. (mostly just helps with some vocabulary), so asking if he used duolingo probably made him assume that you were implying his sentence structure and vocabulary were basic/starter level
i know it's pointless and you don't mean anything by asking if he used duolingo, but the more polite way to inquire about it in American culture would be to say something like "how did you learn the language?" instead. That way they won't assume you're judging their proficiency
You'll notice that the way it teaches you is incredibly inefficient. Instead of learning grammar rules, you just learn different conjugations of verbs as a new vocab word. It's a fun game, but not a good way to learn a language. If someone's really serious about learning a language, my advice would be to take classes at school, or buy a textbook.