I actually thought about using Tianxia for "China" but I had no idea it was already used in Pathfinder.
I doubt the use of it would be illegal since it's a
thing.
Though really I'd prefer to just use "China" for China. Perhaps go the Rockstar route and keep the name of the country/states but have the towns and cities be imaginary.
Like in Grand Theft Auto. It takes place in "America" but it's "Liberty City" instead of New York. Or Los Santos instead of Los Angeles. Etc.