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name: i18n-localization description: Internationalization and localization patterns. Detecting hardcoded strings, managing translations, locale files, RTL support. allowed-tools: Read, Glob, Grep

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> |