Rename zh-tw and zh-cn

[chrisgarrity]

We use the locales CN and TW to determine which character set (traditional vs simplified) to use, not to indicate local language differences. It would be more correct to use zh-Hans for simplified, and zh-Hant for traditional.

However this would require changes across all the repos that use localization and transifex. It might be reasonable to attempt this when we are finally off scratchr2 completely.

[thisandagain]

/cc @kchadha @rschamp

[neilcui0220]

您好，我是一名香港的用戶。
十分感謝閣下提出此要求。