feat(118n): make LanguageMenu reachable from the header

[dcoa]

Description

Moves the language selector into the header and presents it as a globe, per openedx/public-engineering#558 design.

LanguageMenu only lived in the footer, which is off screen on course units and Studio editing views. It now also sits in the header at every viewport, using one component shared by all placements.

It closes #284

Changes

Header placement

• LanguageMenu moves from shell/footer/ to shell/menus/ and is registered in header.desktopRight.v1 and header.mobileRight.v1, after the user/anonymous menu. It stays in the footer.

Globe presentation

• The toggle is a globe icon with the current language's full name at md and up, and the globe alone below. aria-label is "Change language: {language}" at every width.
• Languages are always shown by full localized name.

Mobile menu (below sm)

• The bar's globe steps aside, and a Paragon Collapsible (LanguageMenuCollapsible) is prepended to header.mobileMenuLinks.v1, so it's at the top of the hamburger menu.
• The rows are Paragon MenuItems in a Menu, separated by dividers, with the selected language in the primary color and marked aria-current.
• useLanguageSelection holds the switching logic (list, pending state, optimistic switch, error toast) shared by the dropdown and the collapsible.

Header styling, to match the design reference

• The language toggle and the user menu are borderless (tertiary).
• Sign in is a filled primary button, and Register now comes before it.

Fixes found along the way

• Neither the shell's nor the test-site's index.html had a viewport meta tag, so the mobile header never rendered on phones or under device emulation.
• The mobile menu's FocusOn now has scrollLock={false}. The lock's body[data-scroll-locked] margin collapsed the page when the menu opened. The menu content is also wrapped in the header's Container so it lines up with the bar.

Docs

• docs/how_tos/i18n.rst lists the four widgets and shows how to REMOVE/REPLACE them.

Testing

• Unit tests for LanguageMenu and LanguageMenuCollapsible, covering switching, the pending label, collapsed-by-default, and the error toast.
• Manual, in the test site, signed in and signed out:
• Configure some languages fost the test site (e.g. test-site/src/i18n/messages/{ar,es-419,pt_BR}.json). you can use AI for help.
  • Desktop: globe + name at the far right, after Sign in / the user menu.
  • 576–768px: mobile header with the globe on the bar.
  • Under 576px: no globe on the bar; the hamburger opens with the language collapsible first.
  • Switching language from each placement updates the UI and persists.

Screencast.from.2026-09-18.13-33-46.webm

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Additional information

Supported by Claude

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[arbrandes]

I tested it, and it's fabulous. Thanks, @dcoa!

[openedx-semantic-release-bot]

🎉 This PR is included in version 2.0.0-alpha.14 🎉

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