i18n & Localization

Internationalization (i18n) and Localization (L10n) best practices.

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1. Core Concepts

Term	Meaning
i18n	Internationalization - making app translatable
L10n	Localization - actual translations
Locale	Language + Region (en-US, tr-TR)
RTL	Right-to-left languages (Arabic, Hebrew)

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2. When to Use i18n

Project Type	i18n Needed?
Public web app	✅ Yes
SaaS product	✅ Yes
Internal tool	⚠️ Maybe
Single-region app	⚠️ Consider future
Personal project	❌ Optional

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3. Implementation Patterns

React (react-i18next)

import { useTranslation } from 'react-i18next';

function Welcome() {
  const { t } = useTranslation();
  return <h1>{t('welcome.title')}</h1>;
}

Next.js (next-intl)

import { useTranslations } from 'next-intl';

export default function Page() {
  const t = useTranslations('Home');
  return <h1>{t('title')}</h1>;
}

Python (gettext)

from gettext import gettext as _

print(_("Welcome to our app"))

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4. File Structure

locales/
├── en/
│   ├── common.json
│   ├── auth.json
│   └── errors.json
├── tr/
│   ├── common.json
│   ├── auth.json
│   └── errors.json
└── ar/          # RTL
    └── ...

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5. Best Practices

DO ✅

• Use translation keys, not raw text
• Namespace translations by feature
• Support pluralization
• Handle date/number formats per locale
• Plan for RTL from the start
• Use ICU message format for complex strings

DON'T ❌

• Hardcode strings in components
• Concatenate translated strings
• Assume text length (German is 30% longer)
• Forget about RTL layout
• Mix languages in same file

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6. Common Issues

Issue	Solution
Missing translation	Fallback to default language
Hardcoded strings	Use linter/checker script
Date format	Use Intl.DateTimeFormat
Number format	Use Intl.NumberFormat
Pluralization	Use ICU message format

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7. RTL Support

/* CSS Logical Properties */
.container {
  margin-inline-start: 1rem;  /* Not margin-left */
  padding-inline-end: 1rem;   /* Not padding-right */
}

[dir="rtl"] .icon {
  transform: scaleX(-1);
}

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8. Checklist

Before shipping:

• All user-facing strings use translation keys
• Locale files exist for all supported languages
• Date/number formatting uses Intl API
• RTL layout tested (if applicable)
• Fallback language configured
• No hardcoded strings in components

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Script

Script	Purpose	Command
scripts/i18n_checker.py	Detect hardcoded strings & missing translations	python scripts/i18n_checker.py <project_path>