Summary

Addresses parts of #300:

• Extract the inlined language-picker SVG out of _layouts/menu.html into a new file assets/img/language.svg, matching the existing <img src="/assets/img/..."> pattern used by logo.svg / ecma-logo.svg. Drops the inline role="button" (the new <img> carries its own role and gets a localized alt).
• Remove the stray <!-- Copyright © 2015 Apple Inc. All rights reserved. --> comment and redundant <?xml version="1.0" encoding="utf-8"?> preamble / version="1.1" attribute from assets/img/menu-down.svg.
• Update _sass/_page-menu.scss to target .menu-link__language img instead of the now-removed inline svg.

Left for follow-up

• The logo.svg viewBox concern raised in inconsistent use of language selector icon & other SVG asset problems #300 — a quick eyeball of the rendered logo before/after didn't show an obvious problem, so I didn't change it. Worth a separate PR with a clear visual diff.
• Deeper SVG minification (e.g. SVGO pass over ecma-logo.svg, logo.svg) — left as a follow-up to keep this PR focused.
• Most locales' site.json files don't define navigation.language.title, so the <img alt> ends up empty for them. That mirrors the prior behavior of the .menu-link__language-text span and is a translation-data gap, not an asset issue.

Test plan

• npm run lint passes
• npm run build succeeds; rendered _site/index.html, _site/en/index.html, _site/zh-Hans/index.html all reference /assets/img/language.svg via <img>
• Did not commit unrelated assets/css/style.css regen (sass version bump produced unrelated diffs)
• Visual check on a deploy preview that the language icon still renders at 24px in the desktop menu and that the menu-down chevron is unchanged