react-intl ガイド：React 国際化設定

import React from 'react';
import ReactDOM from 'react-dom/client';
import { IntlProvider } from 'react-intl';
import App from './App';
import enMessages from './messages/en.json';
import deMessages from './messages/de.json';

const messages: Record<string, Record<string, string>> = {
  en: enMessages,
  de: deMessages,
};

// Detect locale from browser or your routing layer
const locale = navigator.language.split('-')[0] || 'en';

ReactDOM.createRoot(document.getElementById('root')!).render(
  <React.StrictMode>
    <IntlProvider locale={locale} messages={messages[locale] || messages.en}>
      <App />
    </IntlProvider>
  </React.StrictMode>
);

// messages/en.json
{
  "app.greeting": "Hello, {name}!",
  "nav.home": "Home",
  "nav.about": "About",
  "nav.settings": "Settings",
  "cart.itemCount": "{count, plural, one {# item} other {# items}} in your cart"
}

// messages/de.json
{
  "app.greeting": "Hallo, {name}!",
  "nav.home": "Startseite",
  "nav.about": "Über uns",
  "nav.settings": "Einstellungen",
  "cart.itemCount": "{count, plural, one {# Artikel} other {# Artikel}} in Ihrem Warenkorb"
}

import { FormattedMessage } from 'react-intl';

function Greeting({ userName }: { userName: string }) {
  return (
    <div>
      <h1>
        <FormattedMessage
          id="app.greeting"
          values={{ name: userName }}
        />
      </h1>
      <nav>
        <a href="/"><FormattedMessage id="nav.home" /></a>
        <a href="/about"><FormattedMessage id="nav.about" /></a>
      </nav>
    </div>
  );
}

import { useIntl } from 'react-intl';

function SearchBar() {
  const intl = useIntl();

  return (
    <input
      type="search"
      placeholder={intl.formatMessage({ id: 'search.placeholder' })}
      aria-label={intl.formatMessage({ id: 'search.ariaLabel' })}
    />
  );
}

// useIntl also gives you formatNumber, formatDate, formatRelativeTime:
function PriceTag({ amount, currency }: { amount: number; currency: string }) {
  const intl = useIntl();
  return (
    <span>{intl.formatNumber(amount, { style: 'currency', currency })}</span>
  );
}

import { FormattedMessage } from 'react-intl';

// Message: "By signing up, you agree to our <link>Terms</link>."
// Key: "signup.terms"
// Value: "By signing up, you agree to our <link>Terms</link>."

function SignUp() {
  return (
    <FormattedMessage
      id="signup.terms"
      values={{
        link: (chunks) => <a href="/terms" className="underline">{chunks}</a>,
      }}
    />
  );
}

# Install the CLI
npm install -g @formatjs/cli

# Extract messages from source code into a JSON file
formatjs extract 'src/**/*.tsx' --out-file messages/en.json --id-interpolation-pattern '[sha512:contenthash:base64:6]'

# Or use explicit IDs (recommended):
formatjs extract 'src/**/*.tsx' --out-file messages/en.json

# Compile messages for production (optional, improves perf)
formatjs compile messages/en.json --out-file compiled/en.json
formatjs compile messages/de.json --out-file compiled/de.json

// ICU MessageFormat syntax — react-intl uses this natively
// English
{
  "cart.itemCount": "{count, plural, one {# item} other {# items}} in your cart",
  "inbox.unread": "You have {count, plural, =0 {no unread messages} one {# unread message} other {# unread messages}}"
}

// Arabic — 6 plural forms
{
  "cart.itemCount": "{count, plural, zero {لا عناصر} one {عنصر واحد} two {عنصران} few {# عناصر} many {# عنصرًا} other {# عنصر}} في سلتك"
}

// Japanese — 1 form (other)
{
  "cart.itemCount": "カートに{count}個の商品があります"
}

// Gender-dependent messages using ICU select
{
  "user.greeting": "{gender, select, male {He} female {She} other {They}} liked your post.",
  "user.invitation": "{role, select, admin {You can manage all settings.} editor {You can edit content.} other {You can view content.}}"
}

// Usage:
<FormattedMessage
  id="user.greeting"
  values={{ gender: user.gender }}
/>

my-react-app/
├── messages/
│   ├── en.json              # Source of truth (English)
│   ├── de.json              # German
│   ├── ja.json              # Japanese
│   └── es.json              # Spanish
├── compiled/                # Optional: compiled messages for prod
│   ├── en.json
│   └── ...
├── src/
│   ├── main.tsx             # App entry with IntlProvider
│   ├── App.tsx
│   └── components/
│       ├── Greeting.tsx      # Uses FormattedMessage
│       └── SearchBar.tsx     # Uses useIntl
└── package.json