Fix RangeError from HTTP quality values in navigator.languages

Duo Sentry Insights Botrequested to merge
duo/fix/625056-invalid-language-tag-quality-values into master

What does this MR do and why?

Fixes a RangeError: Invalid language tag: en;q=0.9 thrown by Intl.DateTimeFormat when navigator.languages contains HTTP Accept-Language quality values (e.g. en;q=0.9) instead of plain BCP 47 language tags.

The getPreferredLocales() function in locale/index.js passes navigator.languages entries directly to Intl.DateTimeFormat. The spec says navigator.languages should contain BCP 47 tags, but some browser environments (observed in Chrome 145) include quality-value suffixes like ;q=0.9. These are not valid BCP 47 and cause Intl.DateTimeFormat to throw a RangeError, breaking date formatting across all pages — most visibly in time_ago_tooltip.vue tooltips.

The fix strips everything from the first semicolon in each locale string before using it, converting en;q=0.9 to en and en-US;q=1.0 to en-US.

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/625056

References

Screenshots or screen recordings

N/A — logic-only fix, no UI change.

How to set up and validate locally

  1. Open browser DevTools console on any GitLab page with a TimeAgoTooltip (e.g. any issue list).
  2. Override navigator.languages to include quality values: 
    Object.defineProperty(navigator, 'languages', { get: () => ['en;q=0.9'] });
  3. Hover over a relative timestamp — before this fix a RangeError is thrown; after the fix the tooltip renders correctly.

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