Chinese and Japanese characters are not shown properly on iOS
If you are talking about the form of the characters not being correct, so you are seeing the Korean character variants instead of the Chinese ones, these are in fact by default determined by which language is highest on the list in Settings > General > Language.  You would have to make Chinese higher than Korean in that list to get the Chinese forms displayed.

Below are some images of those characters Japanese, Chinese, and Korean forms (Korean at the bottom).



While you may be seeing Korean forms on your device, people you send text to will see the forms dictated by the settings on their own machine.  So a recipient of your email or text who is using a Chinese system will see the Chinese forms.

Of course if you are using an app where you can choose the font independently of the system, such as Pages, then you can make sure you choose a Chinese or Japanese font instead of a Korean one to get the forms you want.