Fix web semantics language tags for locales with country codes - #195258

Summary

Use Locale.toLanguageTag() when writing the web semantics lang attribute. A locale constructed as Locale('es', 'US') now produces the BCP 47 tag es-US instead of es_US, allowing browsers and assistive technology to recognize the language correctly.

Fixes #194688

Verification

• The regression test fails with the old implementation: expected es-US, actual es_US.
• Focused Chrome web semantics suite: all 189 tests passed.
• Targeted dart analyze: no issues found.
• dart format --set-exit-if-changed and git diff --check passed.

Contributor review completed on 2026-10-26, and independent spec and standards reviews found no actionable findings. The PR remains a draft because GitHub denied the ready-for-review state change.

Pre-launch Checklist

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• All existing and new tests are passing.