oEmbed iframe: pass language parameter (hl) to provider URLs to match page language (SC 3.1.2)

Problem/Motivation

On multilingual Drupal sites, oEmbed media (YouTube, Vimeo, etc.) is embedded via a sandboxed iframe page. The provider player loads in its default language regardless of the Drupal page language. There is no mechanism in Drupal core to pass a language hint to the embedded player.

This was surfaced in a BOSA accessibility audit (Belgian federal digital accessibility authority), which flagged:

The language of the <iframe> element does not match the language of the current page. The lang attribute value of the YouTube player does not follow the language of the current page. This can be modified via the YouTube IFrame Player API settings (parameter: hl).

This is distinct from issue #3593533 (which adds `lang` to the Drupal HTML wrapper page to address SC 3.1.1). The concern here is the player UI language itself — a SC 3.1.2 (Language of Parts, Level AA) consideration.

YouTube exposes an `hl` parameter on its oEmbed endpoint:

https://www.youtube.com/oembed?url=VIDEO_URL&hl=fr

When present, YouTube returns an iframe src that includes `hl=fr`, which sets the player interface language and influences the default subtitle/caption track.

Vimeo similarly supports a `locale` parameter on its oEmbed endpoint.

Currently, Drupal's `UrlResolver::getResourceUrl()` does not pass any language parameter to provider endpoints, so the player always loads in its default language.

Steps to reproduce

1. Install Drupal with the Media module, set the default language to English, and add a second language (e.g. French).
2. Embed a YouTube video via Media/oEmbed in a node.
3. Visit the node in French (`/fr/node/...`).
4. Open the oEmbed iframe URL directly (`/fr/media/oembed?url=...`).
5. Observe: the YouTube player UI is not in French.
6. Run an automated accessibility audit on the page.

Current behavior

`UrlResolver::getResourceUrl()` builds the oEmbed endpoint URL with only `maxwidth` and `maxheight` parameters. No language hint is passed to providers.

The `hook_oembed_resource_url_alter` hook is available (`UrlResolver.php`, line 137), but no implementation in core uses it to inject a language parameter.

Expected behavior

When a user views oEmbed media in French (or any non-default language), Drupal should pass the active content language to the provider's oEmbed endpoint (e.g. `hl=fr` for YouTube), so that the returned player iframe loads in the correct language.

Proposed solution

Implement `hook_oembed_resource_url_alter` in `MediaHooks.php` (or a dedicated hook file) to append the appropriate language parameter to the oEmbed request URL based on the active content language and the provider name.

Since different providers use different parameter names (YouTube: `hl`, Vimeo: `locale`), the implementation would need to be provider-aware. A clean approach would be to check the provider name and map it to the correct parameter.

Note: YouTube's hl parameter accepts BCP 47 language tags (e.g. fr, nl, fr-BE). Drupal's langcode values are generally compatible, but edge cases (e.g. regional variants) may need verification.

Note: YouTube documentation states that browser preferences may override hl. This parameter is a hint, not a guarantee.

Related issues

• #3593533 — Add lang attribute to oEmbed iframe HTML wrapper (SC 3.1.1) — parent issue; kentr suggested creating this separate issue in comment #20.
• #3085545 — Missing title attribute on the oEmbed inner iframe.
• #3152111 — Duplicate iframe titles.

WCAG reference

SC 3.1.2 Language of Parts (Level AA): If the natural language of a passage, phrase, or label in the content can be programmatically determined, it must be. A media player UI in the wrong language fails this criterion for sighted users relying on the player controls.