About :i18n_dir and multi language site

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

> ```yaml
> /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.

Start with the internationalization documentation at the Path section and review the examples using config.language and the theme's language folder. Clarify that en comes from config.language or config.language[0], and that zh-tw is recognized only when zh-tw.yml exists in the theme language folder.

Written by the indexing model from the issue text.