Translation:There isn't either oil or vinegar in the sauce?
Just for those who are studying English..... this translation is pretty awkward English. We would very likely say "There is neither oil nor vinegar in the sauce." In fact, back when I was first learning French, it was helpful for me to think of Neither/Nor and Ni/Ni - easier to remember.
Actually, since this is a question, this is probably the only phrase where "either...or" would be appropriate:
"Is there either oil or vinegar in the sauce?" as the questioner actually wants to know if either of them are present, not if both are present.
Of course, this translation is only accepted as a typo for the incorrect "Is there neither oil or vinegar in the sauce?"
"Is there either oil or vinegar in the sauce?"
A more accurate translation is, "Is there no oil or vinegar in the sauce?" or "Is there neither oil nor vinegar in the sauce?"
"Is there either oil or vinegar in the sauce?" would be rendered as "Il y a de l'huile ou du vinaigre dans la sauce?"
The way duo translates into English, a native English speaker might be induced to ask, "So which is in the sauce, oil or vinegar ." If "neither (one)" is in the sauce, then use neither/nor. If "either (one)" is in the sauce, but not both, then use either/or. (N)either/-for both. Either/+ for one of the two.
The whole section on "ni" is really tortured English. Perhaps someone should rethink how to present this? These almost direct translations from French into English don't work. The sentence above might more realistically be "There's neither oil nor vinegar in the sauce." or using part of the above sentence, "There isn't any oil or vinegar in the sauce?
I just entered the same response as did you, skfl5, and it was not accepted as correct, which it is, of course. It may be a more formal way of speaking, but it is certainly correct (References: the style manuals of both the CBC (Canada's national and top-notch radio and TV broadcaster - download the free CBC Listen app) and The Globe and Mail (Toronto-based national newspaper), as well as the fabulous grammar tome "Eats Shoots and Leaves".)
I'd say the person asking this question is asking with astonishment and/or disbelief, "There is NEITHER oil NOR vinegar in the sauce?" Duo's translation is incorrect asking if one of either oil or vinegar is not in the sauce. Ni/ni means neither/nor, nothing less and nothing more.
I'm with everyone else, Duo is making a total hash of translating "Ni...ni...." into English.
Plus, this sentence is a question [not a statement ] as indicated by the question mark at the end.
It's too time-consuming having to improve upon/correct Duos translations as well trying to learn the French language.