Set Simplified Chinese as Phone Language in Windows phone 8.1 with AT&T Lumia 920

[WPCentral Question]

Hi, I live in USA and I got a Lumia 920 from AT&T 1 year ago. I got the recent 8.1 update on my phone and everything runs good, except that when I tried to use the Chinese version Cortana, it didn't work. I found that it's because the speech language is not matched with the phone display language, so I tried to change my phone display language to Chinese but it just won't allow me to do so. I can add Chinese to the Language list, but it will still use English as my phone language and I just couldn't find a way to change it.

Is it a bug or some kind of restriction that AT&T sets? Any help will be really appreciated!