[i18n] Support Netlify redirects for non en pages

[chalin]

Currently, the Netlify redirects file only processes pages from the default language (en).

• [i18n] Initial multilingual support for redirects with ja example #4687

Followup to 4687:

• Generalize site redirect rules. Add support for more front-matter attributes
  • [i18n] Process non-en redirect directives #4906
  • Handle aliases too?
• Consider ensuring that the language dropdown doesn't include pages that are redirects

Tuning:

• [CI] Prevent default-lang-only redirects from being duplicated #6544