Make new-style chinese language codes works for translation

The old-style chinese language codes used in django 1.4 series are now deprecated, as discussed here: ```https://code.djangoproject.com/ticket/18419```.
Although majority of browsers still use the old-style language codes, some new browsers such as IE11 in Windows 8.1 start to use the new-style names instead of old-style ones which makes the current chinese translations of edX can't work properly under these browsers.
As there is no 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 or other browsers that use the new-style names can view 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.