fall back logic for zh-xx

[SongChen]

"zh-SG", "zh-CN" should fall back to "zh-HANS" (or "zh-CHS")
"zh-TW", "zh-HK", "zh-MO" should fall back to "zh-HANT" (or "zh-CHT")
"zh" is not a valid generic culture.

[coderifous]

Is there an authoritative resource you can point me to that would show the relationships you describe in your comment - potentially for this and other languages? I could potentially add support for other specifically-handled fallbacks as well, assuming these are not the only ones.