I18n support for language-country code

[VincentTam]

I’m found the solution !

Thanks to the project Public Money Public Code.

# config.toml
[Languages.zh-TW]
  languageCode  = "zh-TW"
  languageName  = "正體中文（臺灣）"

While the file name should be foo.zh-tw.md in lower case for the language-country part.