If the attacker had zero knolwedge of this they would have to rely on random collisions. On the other hand, there are about 5000 commonly used Chinese character

If the attacker had zero knolwedge of this they would have to rely on random collisions. On the other hand, there are about 5000 commonly used Chinese characters (estimates will vary) so entropy is going to be fairly high if they knew. The real issue is how different OS and applications handle character encoding and KDF. A good example is WinRAR which does not allow full width CJK characters to be typed in the password field of its UI but one can copy and paste in anything and it will be accepted. Decryption will work on some OS/version combinations but not all.