feat(i18n): nav dropdown language selector

[Spice-Z]

At #50, @antfu implemented i18n support, and said.

The only thing left is nav dropdown, that would be another PR.

I created that!

Working config is like this

...
themeConfig: {
 locales: {
    '/': {
      label: 'English',
      selectText: 'Languages',
    },
    '/zh/': {
      label: '中文',
      selectText: '选择语言',
    },
    '/ja/': {
      label: '日本語',
      selectText: '言語,
    },
  },
}

(To set config about site title, description, and lang, you have to use global locale config. Check #50)

[Spice-Z]

This text also needs i18n

[Spice-Z]

DONE

[kiaking]

Thank you so much for the feature! Looking really good!

I have 2 requests.

Currently, VuePress locale config sits inside themeConfig. Would it be possible to move the new configs under there instead of locales?

{
  themeConfig: {
    locales: {
      '/': {
        label: 'English',
        selectText: 'Languages',
        editLinkText: 'Edit this page on GitHub',
    },
    ...
  }
}

Ref: https://vuepress.vuejs.org/guide/i18n.html#default-theme-i18n-config

2. Can we move i18n dropdown before GitHub repo link at Nav?

[kiaking]

Lets move this one above the GitHub repo nav 🙏

[Spice-Z]

Moved!

[Spice-Z]

@kiaking
I fixed both of your requests!

About first request, I didn't notice vitepress( and vuepress) has two kinds of locales in config...!
(locales is used as Site Level i18n Config, themeConfig.locals is used as Theme i18n Config.)

Now, this selector will use themeConfig.locals's label and selectText

[kiaking]

Thanks a lot! Just one more thing. I found a bug that this PR will not work on configs that does not contain any locales info. Please see the inline comment I made 🙏

[Spice-Z]

@kiaking
Thank you !
Sorry to late commit..., I fixed it!

I have to think of any every case of config, or have to write test code. 😅

[kiaking]

Great! Thanks a lot. Tested at my side and it is working well 🙌