boost: Add AI Catalog translations for supported RHDH locales

Context

Add translations for the Boost AI Catalog frontend.

• Jira: RHIDP-15479
• Prerequisite: PR #4613 must merge before Fullsend is triggered.

Implementation contract

Use the repository and Boost workspace agent instructions.

The authoritative implementation contract is:

workspaces/boost/openspec/changes/ai-catalog-frontend-translations/

Use this single OpenSpec change for the implementation requirements, design decisions, and task checklist. Do not reconstruct scope from unrelated Boost documents or Jira.

Scope

• Add German, Spanish, French, Italian, and Japanese translation files under workspaces/boost/plugins/boost/src/translations/.
• Register all five locales as lazy imports.
• Ensure every locale has the same flattened keys as boostMessages.
• Preserve interpolation placeholders.
• Add or update focused tests for locale key parity and English fallback.
• Add focused Playwright locale coverage:
  • Keep the existing functional suite English-only.
  • Switch the RHDH language through Settings for each supported locale.
  • Assert representative AI Catalog strings render in the selected language.
  • Remove or update the stale English-only comment in playwright.config.ts.
• Add a changeset for @red-hat-developer-hub/backstage-plugin-boost.

Use established translation implementations in other workspaces as structural examples.

Boundaries

• No backend, OGX, or unrelated UI behavior changes.
• No broad E2E refactor or duplication of the complete functional suite across six locales.
• Do not localize the sample lifecycle filter.
• Do not change the public translation API unless a verified missing string requires it.
• Do not rewrite the archived E2E change; new locale coverage belongs to this translation change.

Verification

From workspaces/boost, follow the required dependency setup in AGENTS.md, then run the relevant checks:

• focused Boost plugin tests and linting
• yarn prettier:check
• yarn tsc:full
• yarn test:e2e
• yarn openspec:validate

If public exports or signatures change, also run yarn build:api-reports:only after type-checking and commit the required API report.

OpenSpec closure in the same PR

The implementation PR must close the OpenSpec bookkeeping after the code and tests are complete:

1. Check only tasks that were actually implemented and verified.

2. Archive the completed change without --skip-specs:

   yarn openspec archive ai-catalog-frontend-translations --yes

3. Ensure the canonical specification is updated by the archive operation.

4. Update workspaces/boost/specifications/CURRENT.md so translations are no longer listed as active work.

5. Confirm openspec list no longer lists the translation change.

6. Confirm yarn openspec:validate passes.

Completion means the code, tests, changeset, canonical specification, task state, archive, and CURRENT.md all agree in the same PR.

Start with workspaces/boost/openspec/changes/ai-catalog-frontend-translations/ and workspaces/boost/AGENTS.md, then inspect the existing files under workspaces/boost/plugins/boost/src/translations/ and the locale registration. Run the focused Boost tests and Playwright setup before adding five locale files, parity and fallback coverage, selected-language AI Catalog checks, and the changeset. Done means the listed checks pass and the OpenSpec change is archived with CURRENT.md updated.