Fix i18n current locale

[mtwilliams-code]

Changes

• This will fix the issue with Astro.currentLocale not working correctly in SSR. See issue Astro.currentLocale doesn't work correctly in SSR with fallback #12838

Testing

Adds similar logic to computeCurrentLocale as was used in #12709 to handle SSR manifests with fallback routes.
Welcome feedback from a maintainer on whether there may be a more elegant solution to solve both of these - I dont have enough background on the decision process behind having such different manifests and i18n implementation between dev/SSR/SSG.

Docs

Just corrects a bug and inconsistent behavior across modes. No new docs needed.

[changeset-bot]

🦋 Changeset detected

Latest commit: bab3349

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[codspeed-hq]

CodSpeed Performance Report

Merging #12839 will not alter performance

_{Comparing mtwilliams-code:fix-i18n-currentLocale (bab3349) with main (4b7a2ce)}

Summary

✅ 6 untouched benchmarks