Add French / German / Japanese localization

[2214962083]

Summary

CronPulse is an open-source tool with global potential. Adding support for multiple languages — starting with French, German, and Japanese — would make it accessible to a much wider audience and demonstrate the project's commitment to internationalization.

Problem

The CronPulse UI is currently English-only. Non-English-speaking users and teams face a language barrier when using the dashboard, reading alert messages, or viewing public status pages.

Expected Behavior / Deliverable

Phase 1: i18n Infrastructure (if not already in place)

• Set up an i18n framework (e.g., i18next, @formatjs/intl, or a lightweight custom solution).
• Extract all user-facing strings from the UI into translation files.
• Add a language selector in the dashboard settings or header.
• Persist language preference in localStorage or user settings.

Phase 2: Translations

• French (fr) translation file
• German (de) translation file
• Japanese (ja) translation file

Each translation file should cover:

• Dashboard UI (navigation, buttons, labels, tooltips)
• Alert/notification message templates
• Status page text
• Error messages
• Settings page

Implementation Hints

• UI components live in: src/ — audit all components for hardcoded strings.
• Common i18n patterns for this stack:
  • JSON translation files in src/locales/en.json, src/locales/fr.json, etc.
  • A translation function t('key') used in components.
  • Dynamic import of locale files for code splitting.
• Date/time formatting should also be locale-aware (use Intl.DateTimeFormat).
• Cron expression descriptions (e.g., "Every 5 minutes") should also be translated if a library like cronstrue is used (it supports i18n).
• Contributors can pick just one language — this issue can be split across multiple PRs.

Acceptance Criteria

• i18n infrastructure set up with translation function
• All hardcoded English strings extracted to locale files
• Language selector available in the UI
• At least one of: French, German, or Japanese translation complete
• Date/time formatting respects locale
• Notification templates support localization
• Contributing guide for adding new languages documented

Note to Contributors

You do NOT need to translate all three languages in a single PR! Pick the one you are most comfortable with and submit a PR for that language. Each language is a valuable standalone contribution.