I think Sulu was arrived at by looking at a world map and seeing the Sulu Sea. It's not a Japanese name.
It wasn't supposed to be. Sulu wasn't meant to be identified with any specific Asian nation, which is why he was given the name of a sea that abutted multiple nations -- although either Roddenberry read the atlas wrong or it's changed since the '60s, because it's really only two or three countries, mainly the Philippines. The only reason we think of Sulu as Japanese is because they cast a Japanese-American actor in the role, and audiences jumped to conclusions -- as did tie-in authors who went against Roddenberry's "pan-Asian" intentions by playing up Sulu's Japanese heritage and interests in books and comics.