Is "J" pronounced like the "je" in French?
A vocalized "sh" sound basically?
Why is (ben japonli degilim) wrong?
Because japon already means a Japanese person.
So japonlu would mean something like "someone who comes from Japanese" -- it makes no sense.
You realise how compact and efficient turkish is as you painstaking type the very long translation of japon degilim, Çinliyim on your phone's small keyboard