About :i18n_dir and multi language site

[SukkaW]

https://hexo.io/docs/internationalization#Path

/index.html => en
/archives/index.html => en
/zh-tw/index.html => zh-tw

We should mention that en here is config.language or config.language[0](If Array is used at config.language). Also zh-tw things will only be considered as language when the zh-tw.yml is available at theme's language folder.

BTW, it is really hard to create a multi language site using Hexo.