Many Chinese fonts seem to use other fonts for the Latin text. For e.g., Adobe Song Std uses a light version of Minion Pro. What Latin fonts are used in Apple's Chinese fonts, such as STFangsong, STKaiti, STSong, STHeiti, and STXihei? As best as I can tell, STFangsong, STKaiti, and STSong all share the same Latin font.
STSong, STFangsong, and STKaiti use a Garamond clone for Latin characters. STHeiti uses a Century Gothic / Avant Garde clone. (ST stands for SinoType Technology Co. Ltd.)
(OS X 10.7)
This is a shot in the dark. If the ones you mentioned are Serifs, I can imagine the others are done in Sans and in that case most likely Myriad as they do on my Mac. Here is a link that may help you break down what's going on. Using the Chinese language on the Mac OS