That's nasty... A fresh install isn't necessarily clean (directories for user data and preferences are not deleted). I know you said the clean start didn't help, but have you actually tried removing the "Base" and "User Base" directories before re-installing? They should be in /Library/Mathematica and ~/Library/Mathematica on a Mac.
Anyway, I'd really give the edit menu another shot first. Here are screenshots of the relevant menus in the English version to help you navigate (I'm using Windows, but the menus should be similar enough):
(After changing the language, you will get a popup message, the good old "why the beep" window, informing you that you need to restart Mathematica for the changes to take effect. It will also be in Chinese, but just click the only button that it comes with and re-start the program.)
I tested the whole thing for you and changed my language to various languages and back, which worked fine (after a re-start, the menu languages were also translated). And yes, I was brave enough to change it to simplified Chinese as well, and changing back to English worked like a charm. (I can post the screenshots in Chinese if you like...)
