Makes new-style chinese language codes works with django 1.4

The chinese language codes used in django 1.4 are deprecated, as described here: ```https://code.djangoproject.com/ticket/18419```
And recently, IE11 in Windows 8.1 also use the new names instead of old ones, which makes the current translation of edX don't work properly for IE11 in Windows 8.1.
As there is easy way for edX to change a higher version of django, I think we could do some mapping here so that the users who use IE11 in Windows 8.1 can view the correct translations.
Also, this fix will make users who use one of the language codes: ```zh-hk```, ```zh-sg``` and ```zh-mo``` can see the correct translations.