Not quite, I think. "It is going bad" is correct English, but not as a translation for the given German sentence. As I and others have said, "It is going bad" is used to refer to foodstuffs that are going rotten, sour, or in some other way becoming inedible.
"It is going bad" is possible in English, but it means something completely different to "It is going badly". "It is going bad" means that some food item is becoming rotten, whereas "It is going badly" describes how a particular thing is progressing (an exam, or a course of study, for example). I don't think the German "Es geht schlecht." would have anything to do with mouldy food, so I agree that the correct answer should be "It is going badly".
I've heard people say "Es geht schlecht/gut" (maybe more commonly "Schlecht/Gut geht's") for "I'm [not] doing fine" (or rather, "It's / things are going badly/well"), but I don't think learners should memorise that. I'd always go for "Es geht mir schlecht/gut", or, in short, just "Schlecht/Gut".
"Es geht schlecht" is something I'd say when I'm asked how the nordic skiing is going / working out: the snow is too wet and I keep getting stuck.
I can't say, of course, if that's what the English sentence was trying to say, but it doesn't work for the German sentence. Maybe the person who wrote the German sentence misunderstood the English one.
"The banana is going bad" would be "Die Banane wird schlecht": "Iss die Banane, sonst wird sie schlecht!" (Eat the banana, or it'll go bad)
"The banana has gone bad" = "Die Banane ist schlecht" (it is now bad and I can't use it) or "Die Banane ist schlecht geworden" (it has gone bad during these past three days).
I agree. Although it is possible in English correctly to say "It is going bad", it means something quite different. It refers to perishable food that is turning mouldy, sour, or stale!
First there are adverbs in English that German does not have. Therefore it is incorrect to say "It is going bad." How it is going is, in English, an adverb. Bad is not an adverb; therefore, the correct word should be "badly" or "poorly" both of which duolingo said were wrong. Since duolingo says English and Spanish are the languages it "teacher" the most, I hope duolingo does a better job of teaching English than it requires of English speakers learning a foreign language and requires English learners to use adverbs correctly (note the adverb!).
I agree. In addition, as I mentioned above, "It is going bad" means something quite different in English, being used when milk or some other perishable foodstuff is turning mouldy, sour, or stale.
I agree that this should not be so, as do many of the previous comments here which have said the same thing. I, like others, have reported it.
Interesting enough, on this page Duolingo says that the translation should be "it is going bad", but when I entered it, it told me that it was wrong, that I should have used "badly" instead. In California if someone asks me How is it going? We answer "Good" or "Bad", not "Goodly" or "Badly". In this context Duolingo should accept both "bad" and "badly" as options .