i18n-loader: treat missing translation files (404) as expected, not errors

[rudberga]

Fixes #41808

Proposed changes

• Fix: In i18n-loader.js, treat HTTP 404 responses as "no translations available" instead of an error. When a site uses a language that doesn't have a Jetpack translation pack (e.g., Afrikaans for Jetpack Search), the i18n-loader would fetch the translation JSON file, receive a 404, and log console.error("Failed to fetch i18n data: Error: HTTP request failed: 404"). A 404 on a translation file simply means no translations exist for that locale/domain — this is an expected condition, not an error. The fix adds a 404 check that returns silently (matching the existing en_US early return pattern), while preserving error handling for genuine failures (500s, network errors, etc.).
• Tests: Updated the existing "Failed fetch" test (which used 404) to expect silent resolution, and added a new "Failed fetch (500)" test to verify server errors still throw.

Related product discussion/links

• Internal: p1739480350788039/1738683024.939459-slack-C02ME06LF (referenced in issue)

Does this pull request change what data or activity we track or use?

No.

Testing instructions

Before (without fix)

1. Go to https://national-rhinoceros-woodpecker.jurassic.ninja/ (site set to Afrikaans, a language without a Jetpack Search translation pack)
2. Open the browser console (DevTools → Console)
3. Trigger Instant Search (click the search bar / search widget)
4. Observe the console error:

   Failed to fetch i18n data: Error: HTTP request failed: 404
   

After (with fix)

1. Go to https://lovely-exceptional.jurassic.ninja/ (test site with fix applied, also set to Afrikaans)
2. Open the browser console and trigger Instant Search
3. Observe: the console.error("Failed to fetch i18n data: ...") is gone
4. The 404 still appears in the Network tab (expected — the browser logs all HTTP requests), but the noisy JS error is eliminated
5. Search still works correctly with English fallback strings

What was tested

• Verified fix eliminates the console.error on an Afrikaans (af) site without a translation pack
• All 13 unit tests pass (pnpm jetpack test js packages/assets)
• Confirmed other HTTP errors (500) still throw as expected (new test)
• Ran /jetpack-review-pr — passed with no blockers or suggestions

What was not tested

• Other languages without translation packs (expected to behave the same — the fix is locale-agnostic)
• Jetpack Search plugin standalone (tested via Jetpack core plugin which shares the same i18n-loader)

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the fix/i18n-loader-404-console-error branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/i18n-loader-404-console-error

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/i18n-loader-404-console-error

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/assets/src/js/i18n-loader.js	29/29 (100.00%)	0.00%	0 💚

Full summary · PHP report · JS report

[rudberga]

Linear ref: SEARCH-188

[Copilot]

Pull request overview

Updates the assets package i18n loader to treat missing translation JSON files (HTTP 404) as an expected “no translations available” outcome, reducing noisy console errors for locales without translation packs.

Changes:

• Return early (no error) when the translation fetch responds with HTTP 404.
• Update unit tests to expect silent resolution on 404 and add coverage for non-404 server errors (500).
• Add a changelog entry documenting the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
projects/packages/assets/src/js/i18n-loader.js	Treats 404 translation fetch responses as a non-error early return while preserving error behavior for other non-OK responses.
projects/packages/assets/tests/js/i18n-loader.test.js	Updates the prior 404 “failed fetch” test to resolve and adds a new test ensuring 500 still rejects.
projects/packages/assets/changelog/fix-i18n-loader-404-console-error	Adds a patch-level changelog entry for the 404 handling change.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[rudberga]

Adding @anomiex as reviewer as you were mentioned in the GH Issue as being the one who investigated this. Please let me know if I should request from someone else, thank you! :)