Make index page of "Overviews" translatable again and add its Japanese translation

[exoego]

Closes #938
cc: @eed3si9n @dongxuwang

The index page of "Overviews" had been redesigned several years ago, without consideration for translation.

This PR makes the redesigned overviews.html template works for other languages.

All actions contributors need to do are

1. Add their lang code in languages property of /_overviews/index.md
2. Add theire _{lang/overviews/index.md based on _overviews/index.md.
   • Ensure layout: overviews. overviews uses a redesigned theme.
   • Drop languages property and add language: {lang} instead.
3. Add /_data/overview-{lang}.yml based on /_data/overview.yml
   • Just keep urls in the file as same as English version. overviews.html will generate automatically a link: if the page has {lang} translated version, the generated link will be a translated version. Otherwise English version.

See how it works in Japanese.

[eed3si9n]

In the screen capture, I see the chapter names of Collection overview in English, like Arrays. Is that correct?

[exoego]

In the screen capture, I see the chapter names of Collection overview in English, like Arrays. Is that correct?

Forgotten to translte.
Fixed in b00eb9f