Translation:Yesterday the temperature was plus one.
On a side note, here's a comic about how Russian uses 'plus' and 'minus':
— Пого́да ска́чет, сейча́с опя́ть плюс. 'The weather is changing quickly (=is jumping), it's plus [=above zero] again now.'
— Тепло́! Но в плю́се есть свои́ ми́нусы. 'It's warm! But there are some minuses [=downsides] in plus [=in temperature above zero].'
— Да уж, весь снег преврати́лся в грязь! 'Well, all the snow turned into mud!'
— А в ми́нус грязь застыва́ет! 'And in minus [=when the temperature is above zero], the mud freezes!'
— И э́то плюс! 'And this is a plus [=an upside]!'
My understanding is that the team was comprised of native Russian speakers volunteering their time for free.
We are the beta-testers, to catch English language errors - not that the given translation is incorrect in this case; "Yesterday the temperature was plus one" is perfectly correct English, even if we're just as likely to say "one degree" (what with positive temperatures being the default assumption unless specified otherwise) as "plus one".
So, maybe just needs a few extra possible translations adding; it'll get there.
We have both Russian and English native speakers on our team, but our course has a ton of sentences, so it was still impossible to include all the translations for each sentence that users can possibly come up with. I added a few more alternatives to the English translation; hopefully this will make it easier for people to get this question correct, without having to guess the 'right' answer. :)
I think it's important to understand that in Russia temperatures change between +30° and −30° a lot during the year. We get a lot of below zero temperatures (more than UK which also uses Celcius). In spring and autumn temperature sometimes hops between below and above zero a few times during the day. So naturally there needs to be a quick and easy way to explain temperatures and indicate whether it's below or above zero.
In Russian we can't say "one above" or "one below" because we can't have a hanging preposition. "One above zero" and "one below zero" is just too long for every day speech. So - we got plus one and minus one. :)
And by the way if context allows we can omit "plus" and "minus" too. For example when you're looking at the thermometer in the middle of February (and your house is buried under heaps of snow),
— Сколько градусов?
— Двадцать пять.
Yes, this sounds pretty natural. In informal speech, it's also common to leave the sign out («вчера́ была́ температу́ра оди́н гра́дус»), and in formal speech, you could say «оди́н гра́дус вы́ше/ни́же нуля́» 'one degree above/below zero'.
In English I think it's safe to say that most would leave out the "temperature" part of this sentence. "It was 1 yesterday" sounds perfectly clear and unambiguous to me; this structure is about temperature by default. Is there an equivalent along those lines in Russian? More/less common than with explicit mention of "температура"?
As much as I'd say "one Celsius", which is an absurd expression for the vast majority of the world's population, unless you're in a lab working in the Kelvin scale. The point is that there are many ways of saying what the Russian say using "plyus" and "minus" and Duolingo should accept many more of them and not waste our time.
I had no idea how to word this naturally in English, and went with the most literal translation I could think of: "Yesterday the temperature was plus one," which was accepted. In American English, I'd probably express this as: "Yesterday it was one degree above/one degree above freezing," which leaves whether it was Fahrenheit or Celsius unspecified; or "Yesterday it was one degree Fahrenheit/Celsius," where "above" is implied (because "one below" would have to be specified).