"We are all humans."
Translation:We zijn allemaal mensen.
No, "alle" would refer to the noun that comes after it: it behaves like an adjective. So "alle mensen" would be all of the humans in existence. While that would technically fit as a translation, the sentence wouldn't make too much sense.
"Allemaal", on the other hand, behaves like an adverb. In this case, it refers to subject "we" to mean all of us.