I18N Date Patterns

i18n and Localization Patterns

Overview

This skill provides comprehensive guidance for implementing internationalization in React applications. It ensures ALL user-facing strings, date displays, currency, lists, and time calculations are locale-aware.

When to use this skill:

• Adding ANY user-facing text to components
• Formatting dates, times, currency, lists, or ordinals
• Implementing complex pluralization
• Embedding React components in translated text
• Supporting RTL languages (Hebrew, Arabic)

Bundled Resources:

• references/formatting-utilities.md - useFormatting hook API reference
• references/icu-messageformat.md - ICU plural/select syntax
• references/trans-component.md - Trans component for rich text
• checklists/i18n-checklist.md - Implementation and review checklist
• examples/component-i18n-example.md - Complete component example

Canonical Reference: See docs/i18n-standards.md for the full i18n standards document.

Core Patterns

1. useTranslation Hook (All UI Strings)

Every visible string MUST use the translation function:

import { useTranslation } from 'react-i18next';

function MyComponent() {
  const { t } = useTranslation(['patients', 'common']);
  
  return (
    <div>
      <h1>{t('patients:title')}</h1>
      <button>{t('common:actions.save')}</button>
    </div>
  );
}

2. useFormatting Hook (Locale-Aware Data)

All locale-sensitive formatting MUST use the centralized hook:

import { useFormatting } from '@/hooks';

function PriceDisplay({ amount, items }) {
  const { formatILS, formatList, formatOrdinal } = useFormatting();
  
  return (
    <div>
      <p>Price: {formatILS(amount)}</p>        {/* ₪1,500.00 */}
      <p>Items: {formatList(items)}</p>        {/* "a, b, and c" */}
      <p>Position: {formatOrdinal(3)}</p>      {/* "3rd" */}
    </div>
  );
}

See references/formatting-utilities.md for the complete API.

3. Date Formatting

All dates MUST use the centralized @/lib/dates library:

import { formatDate, formatDateShort, calculateWaitTime } from '@/lib/dates';

const date = formatDate(appointment.date);    // "Jan 6, 2026"
const waitTime = calculateWaitTime('09:30');  // "15 min"

4. ICU MessageFormat (Complex Plurals)

Use ICU syntax in translation files for pluralization:

{
  "patients": "{count, plural, =0 {No patients} one {# patient} other {# patients}}"
}

t('patients', { count: 5 })  // → "5 patients"

See references/icu-messageformat.md for full syntax.

5. Trans Component (Rich Text)

For embedded React components in translated text:

import { Trans } from 'react-i18next';

<Trans
  i18nKey="richText.welcome"
  values={{ name: userName }}
  components={{ strong: <strong /> }}
/>

See references/trans-component.md for patterns.

Translation File Structure

frontend/src/i18n/locales/
├── en/
│   ├── common.json      # Shared: actions, status, time
│   ├── patients.json    # Patient-related strings
│   ├── dashboard.json   # Dashboard strings
│   ├── owner.json       # Owner portal strings
│   └── invoices.json    # Invoice strings
└── he/
    └── (same structure)

Anti-Patterns (FORBIDDEN)

// ❌ NEVER hardcode strings
<h1>מטופלים</h1>                    // Use t('patients:title')
<button>Save</button>               // Use t('common:actions.save')

// ❌ NEVER use .join() for lists
items.join(', ')                    // Use formatList(items)

// ❌ NEVER hardcode currency
"₪" + price                         // Use formatILS(price)

// ❌ NEVER use new Date() for formatting
new Date().toLocaleDateString()     // Use formatDate() from @/lib/dates

// ❌ NEVER use inline plural logic
count === 1 ? 'item' : 'items'      // Use ICU MessageFormat

// ❌ NEVER leave console.log in production
console.log('debug')                // Remove before commit

// ❌ NEVER use dangerouslySetInnerHTML for i18n
dangerouslySetInnerHTML             // Use <Trans> component

Quick Reference

Checklist

See checklists/i18n-checklist.md for complete implementation and review checklists.

Integration with Agents

Frontend UI Developer

• Uses all i18n patterns for components
• References this skill for formatting
• Ensures no hardcoded strings

Code Quality Reviewer

• Checks for anti-patterns (.join(), console.log, etc.)
• Validates translation key coverage
• Ensures RTL compatibility

Skill Version: 1.2.0 Last Updated: 2026-01-06 Maintained by: Yonatan Gross

Related Skills

• a11y-testing - Accessibility testing for internationalized components and RTL layouts
• type-safety-validation - Zod schemas for validating translation key structures and locale configs
• react-server-components-framework - Server-side locale detection and RSC i18n patterns
• focus-management - RTL-aware focus management for bidirectional UI navigation

Key Decisions

Capability Details

translation-hooks

Keywords: useTranslation, t(), i18n hook, translation hook Solves:

• Translate UI strings with useTranslation
• Implement namespaced translations
• Handle missing translation keys

formatting-hooks

Keywords: useFormatting, formatCurrency, formatList, formatOrdinal Solves:

• Format currency values with locale
• Format lists with proper separators
• Handle ordinal numbers across locales

icu-messageformat

Keywords: ICU, MessageFormat, plural, select, pluralization Solves:

• Implement pluralization rules
• Handle gender-specific translations
• Build complex message patterns

date-time-formatting

Keywords: date format, time format, dayjs, locale date, calendar Solves:

• Format dates with dayjs and locale
• Handle timezone-aware formatting
• Build calendar components with i18n

rtl-support

Keywords: RTL, right-to-left, hebrew, arabic, direction Solves:

• Support RTL languages like Hebrew
• Handle bidirectional text
• Configure RTL-aware layouts

trans-component

Keywords: Trans, rich text, embedded JSX, interpolation Solves:

• Embed React components in translations
• Handle rich text formatting
• Implement safe HTML in translations