"She is American."
Translation:Cô ấy là người Mỹ.
người is a classifier meaning 'person'. So, American will be not just 'Mỹ', but 'người Mỹ'
Ok thanks. I guess it's a little bit like when in English you have to say Englishman and Englishwoman.
Indeed. Cô áy là Mỹ would mean "she is America" but even that way it would still need the classifier for country (cuóc).