Same way you make any other game display Chinese characters (or any characters the game does not originally come with). That is to say find the font and edit to support it, if you are going to Chinese with its thousands of characters/glyphs then you might have to alter the encoding as most European languages only do a few hundred characters (or more likely less than 100 if you count both cases)
In the case of Chinese you might want to look at the Japanese version of the game as it will probably support 16 bit characters right from the start (Japanese also uses lots of characters) with the added bonus of a lot of the Japanese Kanji also being the same as Chinese Hanzi, though I am less familiar with what would need to be done there.
If the font format is not known to an existing tool then you get to figure out the format yourself. If the words in the game are in textures and images then you get to edit the images.