What would you say of Bax? English or Irish where his spirit belonged? How about Rachmaninoff who spent much of his life in the US though his heart was very clearly Russian? Schnittke...Russian or German?
Rachmaninoff is another difficult one. He did live in the U.S. most of his adult life. He is buried in the U.S., alongside his wife. He missed the life and culture he was raised in as a youth in Russia, but all that was lost in the Revolution and he vowed he would never return, even when he was asked to return later in life. The compositions he wrote after emigrating to the U.S. were not insignificant -- Variations on a Theme of Paganini, Symphony No, 3, the Symphonic Dances, Piano Concerto No. 4, Variations on a Theme of Corelli.
However, he did not become an American citizen until shortly before his death. And his most acclaimed and influential works, which established him among the great composers of Romantic era music, were completed or drafted before he left Russia.
So, consistent with my personal inclination to think of composers in the context of their works, as much as I would love to claim him as American, I think of Rachmaninoff as a Russian composer who lived in America.
This is debatable: "And his most acclaimed and influential works, which established him among the great composers of Romantic era music, were completed or drafted before he left Russia." Some would say his mature works are masterpieces whereas his early works are derivative (though finely composed).