"Das sind private Informationen."
Translation:This is private information.
It is, but then the translation would have to be something like "Those are private pieces of information".
Just because a plural form doesn't exist in English for a word doesn't mean it can't exist in any other language, and I think that is the case here.
This is right Phil, because the word Imformationen does exist in the German language and maybe in others too. Here Duolingo only accepted "informations" as the correct answer, and this is actually wrong. Just saying, it is no use to teach students things, they are obviously wrong. (Duolingo corrected it now) :-)