How do you say the animals are coming the day after tomorrow?
the same way. Vin = I/they come, I/they are coming
Why it is not correct when i translate ... At the day after ...? I am not native speaker of english and don't see a reason.
We use "at" for times: "at 3 pm," etc. "The day after tomorrow," like "today," "yesterday," etc. take no preposition. We use "on" for days of the week: "on Tuesday," etc.
We use at for exact times in English, like "at 3pm," like Howard said, bor an exact location, like "at church," "at home," but if it's not an exact time, like tomorrow or the day after tomorrow, we do not use at in English
Can't I say: "The animals come in two days" ?
No, because the literal translation is not in two days, poimâine literally means the day after tomorrow.