Uh, it may be a long story to tell.
For short, in django 1.6 or earlier, it uses the language code ```zh-cn``` for ```Simplified Chinese``` and ```zh-tw``` for ```Traditional Chinese```. And you can see that these two codes are region-specific language codes, as ```zh-cn``` is the language code of Mainland China and ```zh-tw``` is the code of Taiwan. But in fact, the ```Simplified Chinese``` is used not only in Mainland China, but also in Singapore, Malaysia and other regions. And similarly, the ```Traditional Chinese``` is used not only in Taiwan, but also in Hongkong, Macau and other regions. So the use of region-specific language codes for region-neutral languages are not a good solution, and new IETF standard recommends that use ```zh-Hans``` for ```Simplified Chinese``` and ```zh-Hant``` for ```Traditional Chinese```, and all the region-specific chinese language codes should be updated by appending the region code after the region-neutral code. For example, ```zh-Hans-cn``` is the new-style replacement of the old-style name ```zh-cn``` and ```zh-Hant-tw``` is the new-style replacement of the old-style name ```zh-tw```.