Although "your birthdays are not in October" is not a normal usage in English, it is not incorrect. I have a birthday every year and they are not in October. Native Polish speakers may understand the Polish sentence to imply singular, but for native English speakers, allowing the literal translation as a correct alternative would help us .
We'd prefer to stick to the normal usage though. We don't want people who are not native English speakers to think that this is a common thing to say in English, after all this course is also taken by people wanting to improve their English. And when a Polish person say "urodziny", we rather don't think "they are every year in October"...