URLs without locales do not do correct locale fallback

[DonalMe]

Summary

We do not include a local in the MDN URLs in the Firefox release notes. The goal is for users in non-en locales following links in the release notes to be correctly redirected to their locale.
This is not working for MDN URLs and leads to a 404 in non-English locales.

URL

Example: https://developer.mozilla.org/docs/Web/API/WebCodecs_API

Reproduction steps

1. Browse in a non-en locale
2. Navigate to a MDN URL without a locale (example https://developer.mozilla.org/docs/Web/API/WebCodecs_API)

Expected behavior

There should be a redirect to your locale (example https://developer.mozilla.org/fr/docs/Web/API/WebCodecs_API)

Actual behavior

404 page

Device

Desktop

Browser

Firefox

Browser version

Stable

Operating system

Mac OS

Screenshot

No response

Anything else?

curl --header "Accept-Language: fr,fr-fr,en-us,en" -LIs https://developer.mozilla.org/docs/Web/API/WebCodecs_API | grep HTTP

Validations

• I have read the Community Participation Guidelines.
• I have verified that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• I have checked that this is a concrete bug. For Q&A open a GitHub Discussion.

[caugner]

@DonalMe Overall, this seems to work as intended: https://developer.mozilla.org/docs/Web/API/WebCodecs_API redirects to https://developer.mozilla.org/fr/docs/Web/API/WebCodecs_API, because this is the first locale in the Accept-Language header that MDN supports, but since the page isn't localized yet, a 404 page is displayed instead, with a link to the English version.

Do you expect to be redirected to en-US instead, because it's the first locale listed in the Accept-Language header for which the page exists?

[DonalMe]

@caugner sorry, I didn't realize this behavior was because the page wasn't localized.
If the redirect works when the page is localized, maybe it should fallback to en-us when it is not localized instead of a 404?

[pascalchevrel]

IMO, MDN should go through all the accept-lang fallback locales through content negociation (https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#the_accept-language_header)