frontend: plugins: Only fetch the active locale for plugin i18n

[r0hansaxena]

Summary

Fixes a flood of 404 requests for plugin translation files. Plugin i18n eagerly fetched a translation.json for every locale a plugin declares, so a plugin declaring many locales whose files aren't present produced a 404 per missing locale on every render, enough to trip rate limiters like CrowdSec. Reported for the Prometheus plugin, but the fix applies to all plugins.

Related Issue

Fixes #4854

Changes

• Plugin i18n now eagerly loads only the active language (plus the en fallback) instead of every declared locale
• Other locales are loaded lazily on language switch
• Plugins that declare no locales are skipped entirely (no probing)
• Added unit tests for the above

Steps to Test

1. Use a plugin that declares multiple locales but ships no translations
2. Check the Network tab filtered by the plugin name
3. Before: one 404 per declared locale. After: only the active language is fetched

Screenshots (if applicable)

Before:

After:

Notes for the Reviewer

• Only changes how the frontend loads plugin translations. If the translation files are missing from the build itself, that's a separate packaging fix.

[k8s-ci-robot]

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[r0hansaxena]

I'll address the failing check

[kubernetes-prow]

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This pull-request has been approved by: r0hansaxena
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