Yes, "w" meaning "in, inside" takes Locative. There are also situations when "w" takes Accusative.
Compare: "mam złamaną kość w nodze" (I have a broken bone in my leg, Locative), and "uderzyłem się w nogę" (I hit myself in the leg, Accusative. not even sure if "in" is the right preposition here).
Oh, btw: in April, in October etc. take Locative, but days of the week and the word "weekend" are exceptions and take Accusative despite using "w".
We like sentences that are a bit strange semantically (especially using animals as the subject of the sentence), but well, there's also grammar :) And I was told by a native, that using Present Continuous makes very little sense here.
Quote 1: "It's weird... It's like I've known you as a vegetarian for a long time and you're telling me that you're going to start eating meat in April."
Quote 2: "The other possibility I can think of is that you're going to eat nothing but meat in April, which is also very unlikely and very weird, besides being unhealthy. :)"
And if I really meant one of those, I'd phrase it differently in Polish ;)
"I am eating meat in April". was not accepted, and I read your explanation for why you think using continuous would be unusual. But If one was previously not eating , and then was to begin, would it not be possible to use continuous? And after mentioning it to a friend, and the friend answered "in May?", and the speaker corrected, saying, "I'm eating meat in April." I don't find that an awkward sentence to be using continuous.
There are usually dozens, if not hundreds of accepted translations for a sentence. Duolingo can only show you one, chosen as the best one, plus if you actually typed the best one, it can show you another translation chosen by the course creators. From what I've seen, course creators tend to choose the second translation to highlight alternative meanings of the words or multiple correct grammar forms, but not always – things that are trivial and should be understood by then are considered minor variations not worth mentioning. The fact that Polish has only one present tense was taught pretty early.