Translation:The island has a population of five million people.
"People" is equivalent to "persons," even though Americans usually say "people."
sou brasileiro, e por que não se pode traduzir como "the island has a population of five million of people"?
You can only say "of" after "million" when it's a pronoun or articles (a, the, this, that, etc): "I have five million of them", "give me 2 million of these papers", etc. If it's just a noun with no article (such as "people"), you cannot say "five million of"
"A ilha tem uma população de cinco milhões de pessoas" is okay.
"A ilha tem uma população de cinco milhões pessoas" is okay.
"A ilha tem uma população de cinco milhões habitantes" is okay.
Is that right?
What do we have to do to work "povo" and "a gente" into the end of the given sentence?
You should use "de" after "milhões" in all instances.
"Povo" and "gente" (because "a gente" = we/us) don't work in this case. It's like a standard collocation to use "milhões de pessoas/habitantes". Maybe because "pessoa" and "habitante" can be used to identify one individual, and "povo/gente" is applied to a group.
when "million" follows a number it is never plural: one million, two million, three million, four million, etc. It can only be plural if there is no number in front: "the island has a population of millions of people"
Is "de pessoas" necessary here in Portuguese? In English, unless we want to specify that we're talking about the population of something other than people, we would probably just say "The island has a population of five million." Would "A ilha tem uma população de cinco milhões" also be considered correct in Portuguese?