test: improve 100% test coverage for i18n and campaign signup page

## Issue Summary

Implemented a comprehensive i18n test suite ensuring 100% test coverage for internationalization configuration, language handling, and fallback behavior.

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## Problem Statement

The i18n module previously lacked complete test coverage, which could lead to:

• Undetected issues in language switching
• Missing validation for fallback behavior
• No verification of missing translation handling
• Risk of breaking interpolation and formatting logic
• Lack of confidence in RTL support and namespace handling

This affected overall application reliability and maintainability, especially in multilingual scenarios.

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## Proposed Solution

• Developed a full test suite using Vitest for i18n module

• Covered all critical areas:

  • Initialization config
  • Language switching (changeLanguage)
  • Language retrieval (getLanguage)
  • RTL detection (isRTL)
  • Missing key handling
  • Interpolation formatting
  • Namespace handling
  • Resource fallback logic

• Implemented:

  • Mocking for localStorage
  • Mocking for console.warn
  • Environment-based behavior (DEV vs PROD)
  • Module isolation using vi.resetModules()

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Test-Driven Development

Acceptance Criteria (Given-When-Then)

Scenario 1: Language Switching

Given the application is initialized with default language
When the user changes language to Telugu (te)
Then the application language should update to "te"
And the selected language should be stored in localStorage

Scenario 2: Fallback for Invalid Language

Given an unsupported language is provided
When changeLanguage is called
Then the language should fallback to "en"

Scenario 3: Missing Translation Key Handling

Given a translation key does not exist
When i18n.t() is called
Then a warning should be logged in DEV mode
And a fallback string should be returned

Scenario 4: RTL Language Detection

Given the current language is Arabic
When isRTL is called
Then it should return true

Scenario 5: Interpolation Formatting

Given a translation uses interpolation formatting
When the value is passed with uppercase format
Then the result should be transformed to uppercase

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### Test Cases

Test ID	Test Description	Precondition	Test Steps	Expected Result	Priority
TC-001	Change language to Telugu	i18n initialized	Call changeLanguage('te')	Language updated & stored	High
TC-002	Invalid language fallback	i18n initialized	Call changeLanguage('fr')	Falls back to "en"	High
TC-003	Missing key handling	DEV mode	Call i18n.t('missing.key')	Warning logged	High
TC-004	RTL detection	Language = 'ar'	Call isRTL()	Returns true	Medium
TC-005	Interpolation uppercase	Value provided	Call i18n.t with format	Returns uppercase	Medium

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### Unit Test Requirements

Components/Functions to Test:

Module/Component	Function	Test Cases
i18n.ts	changeLanguage	TC-001, TC-002
i18n.ts	getLanguage	Default + fallback
i18n.ts	isRTL	RTL + non-RTL
i18n.ts	missingKeyHandler	DEV + PROD
i18n.ts	Interpolation	Formatting cases

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## Implementation Details

Files to Change

File Path	Action	Purpose
src/i18n.ts	Existing	i18n configuration
src/__tests__/i18n.test.ts	Created	Full test coverage

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### Technical Considerations

• Used Vitest for unit testing

• Mocked:

  • localStorage
  • console.warn

• Used vi.resetModules() for isolated module testing

• Ensured no dependency on real browser APIs

• Handled DEV vs PROD behavior explicitly

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## Testing Strategy

Manual Testing

1. Change language from UI
2. Refresh page
3. Verify persisted language
4. Check fallback behavior for unsupported language

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### Automated Testing

Unit Tests:

• ✅ Initialization tests
• ✅ Language switching tests
• ✅ Missing key handling
• ✅ Interpolation formatting
• ✅ RTL detection
• ✅ Resource fallback coverage

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### Test Data

const mockTestData = {
  language: 'te',
  invalidLanguage: 'fr',
  rtlLanguage: 'ar',
  text: 'hello'
};

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## Definition of Done

• Acceptance criteria met
• All test cases passing
• Unit tests written (100% coverage achieved)
• Manual testing completed
• Code reviewed
• No console errors
• Type-safe implementation
• ESLint/Prettier passed
• i18n logic fully validated

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## Additional Context

Notes

• Achieved true 100% coverage including:

  • Edge cases
  • Error handling
  • Fallback scenarios

• Tests ensure high confidence in multilingual support

• Improves maintainability and prevents regression issues