Translation:My wife will have decided by tomorrow.
Here "tomorrow" and "by tomorrow" are both accepted as translations. And many other Duolingo sentences with future perfect tense + time-marking adverbs offer both possibilities for translation: with "by" and without "by" However, for me, the sentences' meanings change pretty significantly, depending on whether you include/do not include the word "by." For example, the sentence "Tomorrow I will have eaten the sandwich" does not mean quite the same thing as "By tomorrow I will have eaten the sandwich." I've spent fifteen minutes trying to draft an explanation of what the differences in meaning in English are, but will spare you that discussion, as it's way too long. I'm suspecting, though, that the Swedish-English translations would be more exact if you were to REQUIRE the "by" in the English translations.
No, versions without by are still accepted and the machine tells me My wife will have decided tomorrow is an accepted answer. so I'm not sure what may have happened. We had gotten a bunch of misspelled suggestions so there are a lot of things that can accidentally go wrong in this one (we don't control what spelling mistakes are accepted or not, an algorithm does that). But the machinery can also be a bit glitchy sometimes.
Looking at the construction of the phrase, is it similar to 'my wife will have come to a decision (by) tomorrow'? Or would 'my wife will come to have decided (by) tomorrow' be more acccurate? I understand what it means, I just find that understanding construction and connections helps me to remember much more easily.