"They are not people."
Translation:Oni nie są ludźmi.
Because that would be like "They are not-people". You have to negate the whole idea of "being people", not just "people".
Nouns after the verb "to be," in this case nie są, take the instrumental case. Ludzie becomes ludźmi.
But that's quite a good question. I imagine that'd be a sentence about aliens. While we don't know anything about them and whether they even have genders, "kosmici" are masculine personal. So are "obcy".
"Ufoludki" is not masculine-personal, on the other hand... at least I think so.
We don't use the 'to' construction after a personal pronoun, we consider it too clumsy.
I typed in "To nie są ludzmi" and instead of correcting me to "Oni nie są ludźmi." it corrected me to "To nie są ludzie" but it feels wrong, shouldn't it be "ludźmi" instead of "ludzie" ?