Mac OS X comes with all languages it supports. So if you buy a Mac in China, you get a Mac capable of having its interface in all supported languages, including English, French, Spanish, Danish, .... .
An update will not void that functionality. I have set my Macs to English, even though I bought them in a non-English speaking country.
To change the language of your GUI, go to System Preferences > Language & Text. This is not Windows, which only seems to come with "localised" versions