I wonder what you call a person who has got 2 nationalities like his dad is chinese and his mum is british ?
A half-cast or what ?
Secondly, when I write the word "double nationality" ,do I need to change y into ies or not ?
Whatever country the person is a citizen of is his/her nationality. Forget where your parents were born. If the father is from China and the mom is from British but the child was born in the United States, the child's nationality is American.
Some people have dual citizenship. I have a friend, for instance, who is a citizen of both Canada and England. But she doesn't regard herself as British. She regards herself as a Canadian, since her primary residence is in Canada. Dual citizenship is probably a case of the individual's choice to identify with the nationality they choose.