i18n and Localization Patterns

Overview

• 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)

• 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

docs/i18n-standards.md

Core Patterns

1. useTranslation Hook (All UI Strings)

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)

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>
  );
}

references/formatting-utilities.md

3. Date Formatting

@/lib/dates

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)

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

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

references/icu-messageformat.md

5. Trans Component (Rich Text)

import { Trans } from 'react-i18next';

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

references/trans-component.md

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

t('namespace:key')

useTranslation

formatILS(amount)

useFormatting

formatList(items)

useFormatting

formatOrdinal(n)

useFormatting

formatDate(date)

@/lib/dates

<Trans>

isRTL

useFormatting

Checklist

checklists/i18n-checklist.md

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

Related Skills

• testing-patterns

  - Comprehensive testing patterns including accessibility testing for i18n

• 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

• accessibility

  - RTL-aware focus management for bidirectional UI navigation

Key Decisions

Capability Details

translation-hooks

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

formatting-hooks

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

icu-messageformat

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

date-time-formatting

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

rtl-support

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

trans-component

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