limit display of non-English content to the top 10 locales

Reference

https://hacks.mozilla.org/2021/03/mdn-localization-in-march-tier-1-locales-unfrozen-and-future-plans/

Summary

On April 29/30, we will stop displaying all except the top 10 non-English locales, which are fr, ru, ja, ko, zh-CN, zh-TW, es, de, pl, and pt-BR.

• Move the other 22 non-English locales out of https://github.com/mdn/translated-content and into the https://github.com/mdn/retired-content repository.

• Update VALID_LOCALES to reflect this change:

  https://github.com/mdn/yari/blob/d4fd6673ad5bdae1647938b427e65e8ea0d1961f/libs/constants/index.js#L1

• Retire the selected 22 non-English locales in Kuma

• Deploy updated versions of the lambda@edge functions that use VALID_LOCALES

• Add redirects for all requests for documents in the 22 dropped locales to their English equivalents.

• Add note/banner to at least the doc pages that explains the reason for the redirect.

[schalkneethling]

@schalkneethling @chrisdavidmills @peterbe @Gregoor If we decide to go with showing some kind of note to those that get redirected from a retired locale, would we need to show that on all of the SPA’s (home page, search page, doc page, settings page, etc.) or just the doc pages?

I reckon just the docs pages would be fine.

[schalkneethling]

100% Agree with @chrisdavidmills suggestion. When we do this, we can reuse the localized notes feature to display the message.