You can change the Region and Language in your phone's settings.
You can also view the app store for another region online by going to http://www.windowsphone.com/[LOCALE]/store .
United States (EN-US) Store: www.windowsphone.com/en-us/store
Russia (RU-RU) Store: www.windowsphone.com/ru-ru/store
British (EN-GB) Store: www.windowsphone.com/...
On WindowsPhone.com, there is a pretty good tutorial on this.
1. On Start, flick left to the App list, then tap Settings > Keyboard.
2. Select all the languages you'd like to be able to use from your keyboard.
The next time you use your keyboard, you'll see a button next to the Space bar
that indicates which language you're currently using on ...
Cortana is available in China. But the language must be set to Chinese (simplified) or 中文 (简体). Bing translate indiactes that you have set the language to China (People's Republic of China) or 中国（中华人民共和国） which is not supported by Cortana. In case you want to know Chinese (simplified) is located next to China (People's Republic of China).
So go to Settings -...
A couple people have reported the same issue here: http://forums.wpcentral.com/windows-phone-7/177931-marketplace-wrong-language.html
If it was a mistake in the update, there is probably nothing you can do to fix it until a new update comes out.