Internationalization (i18n) & Localization

Table of Contents

• Overview
• When to Use
• Quick Start
• Reference Guides
• Best Practices

Overview

Comprehensive guide to implementing internationalization and localization in applications. Covers message translation, pluralization, date/time/number formatting, RTL languages, and integration with popular i18n libraries.

When to Use

• Building multi-language applications
• Supporting international users
• Implementing language switching
• Formatting dates, times, and numbers for different locales
• Supporting RTL (right-to-left) languages
• Extracting and managing translation strings
• Implementing pluralization rules
• Setting up translation workflows

Quick Start

Minimal working example:

// i18n.ts
import i18next from "i18next";
import Backend from "i18next-http-backend";
import LanguageDetector from "i18next-browser-languagedetector";

await i18next
  .use(Backend)
  .use(LanguageDetector)
  .init({
    fallbackLng: "en",
    debug: process.env.NODE_ENV === "development",

    interpolation: {
      escapeValue: false, // React already escapes
    },

    backend: {
      loadPath: "/locales/{{lng}}/{{ns}}.json",
    },

    detection: {
      order: ["querystring", "cookie", "localStorage", "navigator"],
      caches: ["localStorage", "cookie"],
    },
  });
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the references/ directory:

Guide	Contents
i18next (JavaScript/TypeScript)	i18next (JavaScript/TypeScript)
React-Intl (Format.js)	React-Intl (Format.js)
Python i18n (gettext)	Python i18n (gettext)
Date and Time Formatting	Date and Time Formatting
Number and Currency Formatting	Number and Currency Formatting
Pluralization Rules	Pluralization Rules
RTL (Right-to-Left) Language Support	RTL (Right-to-Left) Language Support
Translation Management	Translation Management
Locale Detection	Locale Detection
Server-Side i18n	Server-Side i18n

Best Practices

✅ DO

• Extract all user-facing strings to translation files
• Use ICU message format for complex messages
• Support pluralization correctly for each language
• Use locale-aware date/time/number formatting
• Implement RTL support for Arabic, Hebrew, etc.
• Provide fallback language (usually English)
• Use namespaces to organize translations
• Test with pseudo-localization (ääçćëńţś)
• Store locale preference (cookie, localStorage)
• Use professional translators for production
• Implement translation management workflow
• Support dynamic locale switching
• Use translation memory tools

❌ DON'T

• Hardcode user-facing strings in code
• Concatenate translated strings
• Assume English grammar rules apply to all languages
• Use generic plural forms (one/many) for all languages
• Forget about text expansion (German is ~30% longer)
• Store dates/times in locale-specific formats
• Use flags to represent languages (flag ≠ language)
• Translate technical terms without context
• Mix translation keys with UI strings
• Forget to translate alt text, titles, placeholders
• Assume left-to-right layout