"Yes, I am Japanese?"
"A cheerful heart is good medicine."
hai, nihon-jin desu
That's "Yes, [I/she/he/we/they] am/is/are Japanese (person/people)." The subject is implied based on the situation. There's a lot of different ways to say "its something Japanese" but maybe "Sore wa nihon-teki na mono desu." (that might be more like "Japanese style thing") or "Sore wa nihon tsukutta mono desu." (more like "made in Japan thing")
Pop music is about stealing pocket money from children. - Ian Anderson