"We are American."
Translation:Chúng tôi là người Mỹ.
Mỹ is the shorthand version of Mỹ Lợi Gia, a Chinese loanword 美利加 (Mei Li Jia) that in turn derived from 'America'. In both languages, it's shortened to just the first syllable (Nước Mỹ and Meiguo), which just happened to be the second syllable in America, 'Mer'.
Generally, from experience, it means the United States.