feat(i18n): allow `page.lang` to override `site.lang`

[neilboyd]

Type of change

• [x] New feature (non-breaking change which adds functionality)

Description

Allow lang on page/post to override site.lang.

This is useful to create folders (collections) of different languages. For example, I have folders _en, _ru and _tr (relative urls en, ru and tr), and then define these as collections in _config.xml:

collections:
  en:
    output: true
  ru:
    output: true
  tr:
    output: true

defaults:
  - scope:
      path: ""
      type: en
    values:
      lang: en
  - scope:
      path: ""
      type: tr
    values:
      lang: tr-TR
  - scope:
      path: ""
      type: ru
    values:
      lang: ru-RU

It's also possible to just put lang: ru-RU directly into the page front matter.

Additional context

I removed usage of site.alt_lang in the html lang tag of default layout because I'm not sure what that was for. If there's a reason for it then I could put it back.

I don't exactly know what js-selector.html is doing with lang, so I couldn't test it, but the change seems obvious.