fix(transloco): avoid duplicate fallback lang request when already cached

[arturovt]

When useFallbackTranslation is enabled, loading a secondary language (e.g. 'es') would internally call getFallbacksLoaders, which invokes resolveLoader directly — bypassing the service cache. This caused a duplicate HTTP request for the fallback lang whenever it had already been fetched (e.g. as the default lang).

Fix: before delegating to getFallbacksLoaders, check whether the fallback lang's observable is already in the cache. If it is, reuse it via a mapped forkJoin instead of issuing a new network request.

Adds a regression test covering the case where fallbackLang matches defaultLang and a second language is loaded afterward.

Closes #625

PR Checklist

• The commit message follows our guidelines: https://github.com/jsverse/transloco/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix

Does this PR introduce a breaking change?

• No

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Improved translation loading performance by preventing redundant loader calls when fallback languages are already cached.

• Tests

  • Added test coverage for fallback language caching behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: be9440a9-a661-4795-aa52-b40bb7f74085

📥 Commits

Reviewing files that changed from the base of the PR and between bd284ec and 65aa194.

📒 Files selected for processing (2)

• libs/transloco/src/lib/tests/service/missingHandler.spec.ts
• libs/transloco/src/lib/transloco.service.ts

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📝 Walkthrough

Walkthrough

Modified TranslocoService.load to check the cache before constructing fallback loaders. When a fallback translation already exists in cache, it reuses the cached observable instead of calling getFallbacksLoaders. A test case was added to verify this optimization prevents duplicate loader invocations.

Changes

Cohort / File(s)	Summary
Fallback Translation Cache Optimization libs/transloco/src/lib/transloco.service.ts, libs/transloco/src/lib/tests/service/missingHandler.spec.ts	Implemented cache check in TranslocoService.load to prevent redundant loader calls when fallback translations are already cached. Uses forkJoin with cached observable when available, otherwise falls back to prior getFallbacksLoaders behavior. Added test case to verify no extra loader invocations occur when loading additional active languages with a cached fallback.

Suggested reviewers

• shaharkazaz

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Fun fact: Did you know? In i18n, fallback languages follow a cascade pattern—if a translation isn't available in the requested language, the system gracefully falls back to alternatives. This is why many apps default to English as the ultimate fallback. In Portuguese, the word "idioma" means language!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: avoiding duplicate fallback language requests when the translation is already cached.
Description check	✅ Passed	The PR description includes the problem statement, solution, test coverage, and addresses all required checklist items from the template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-625

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commit: 65aa194

[cubic-dev-ai]

No issues found across 2 files