Rename Locale for Chinese (China) to zh-CN

[lidel]

Received some useful hints from our friends at Transifex and I think this approach is the way to go forward.

This PR applies revised naming convention from ipfs/ipfs-gui#50 (comment)

• Renames Chinese locale from zh to zh-CN, which is supported by i18next and match value from Accept-Language:

  

I smoke tested this PR and strings that are already translated rendered correctly:

ps. In the future we can add pt-BR and en-GB using the same convention :)

[lidel]

This is basically #784 but with changed code, so safe to merge.