I've seen someone using the word "peoples" today, with a vehement claim that it "is NOT grammatically incorrect". I searched on the net, but I got links (such as this) which suggests it is correct. Either way, is there a reference which could settle this issue ?
This is an entry from OALD online:
3 [countable] all the persons who live in a particular place or belong to a particular country, race, etc
the French people
the native peoples of Siberia
So, when used in this sense, the word "people" has a plural form and it is grammatically correct.