How To Use React-Intl: internationalize your React app

This short tutorial assumes you know about ES6 syntax.

Get started

yarn add react-intl or npm i --save react-intl.

Create a messages file

This file should contain every message for your app.
It has an object of (locale, messages) couples:

export default {
  'en-GB': {
    // We're using dots for nesting and camelCase for each name
    'homepage.welcome': 'Welcome!',
    'homepage.subscribeHeader': 'Subscribe Now',
    // You can use placeholders using curly braces ({variable})
    'user.hello': 'Hi {username}, how are you?',
    // ...and also HTML
    'user.signIn': 'Please, <em>sign in</em>'
  },
  'fr-FR': {
    'homepage.welcome': 'Bienvenue !',
    'homepage.subscribeHeader': 'Abonnez-vous',
    'user.hello': 'Bonjour {username}, comment ça va ?',
    'user.signIn': 'Veuillez <em>vous connecter</em> s\'il vous plaît'
  }
};

Each message should be associated to a unique ID that will be required later (e.g. homepage.welcome).
We'll now assume that these messages are stored in a messages.js file.

Import messages

At the root of your app, import your messages file:

import messages from './messages';

Import languages for react-intl

For each language you're using in your application, you'll need one additional line on your same app index file:

// If we're using English and French ('en', 'fr')
import en from 'react-intl/locale-data/en';
import fr from 'react-intl/locale-data/fr';

// But if you need Spanish for example, you'd then need to import
import es from 'react-intl/locale-data/es';

And then tell react-intl to use them (we'll import addLocaleData right after):

addLocaleData([...en, ...fr]);

Wrap this up

It's now time to provide the messages to all our components.
Here, we'll need both IntlProvider and addLocaleData (from the previous step):

// Paste this before the previous imports
import { addLocaleData, IntlProvider } from 'react-intl';

Look up for the user's locale:

const locale = (navigator.languages && navigator.languages[0])
               || navigator.language
               || navigator.userLanguage
               || 'en-US';

And add the provider to your app:

ReactDOM.render((
  <IntlProvider locale={locale} messages={messages[locale]}>
    {/* Previous code... */}
  </IntlProvider>
, document.getElementById('root'));

As you can see, we're telling react-intl what locale and messages should be used, through its props.

Insert messages in your app

You can now use every message you put into your messages.js file, inside your app.

• FormattedMessage: for normal messages:

import { FormattedMessage } from 'react-intl';

class HomePage extends React.Component {
  render() {
    return (
      <div>
        <h1>
          <FormattedMessage id="homepage.welcome" />
          <FormattedMessage id="user.hello" values={{ username: 'Arnaud' }} />
        </h1>
      </div>
    );
  }
}

• FormatedHTMLMessage: for every message containing HTML syntax:

<FormattedHTMLMessage id="user.signIn" />

• FormattedDate & FormattedTime: yup, for every date/time related message (they both need a Unix timestamp in ms):

<FormattedDate
  value={new Date(1501257521)}
  year="numeric"
  month="long"
  day="numeric"
/>

<FormattedTime
  value={new Date(1501257521)}
  year="2-digit"
  month="2-digit"
  day="2-digit"
/>

• FormattedRelative: 10 days ago..., and so on:

<FormattedRelative value={new Date(1501257521)} />

• FormattedNumber: prices, random numbers, ...:

<FormattedNumber
  value={64.42}
  style="currency"
  currencyDisplay="symbol"
  currency="USD"
/>

• otherwise, you're probably looking for a non-component message, a simple string you want to be returned:

import { injectIntl } from 'react-intl';

// 'intl' will be available among the component's props, after the injection
function HelloWorld({ intl, ...props }) {
  const placeholder = intl.formatMessage({
    id: 'random.placeholder', // This ID could be anything else as well
  });
  
  return <textarea placeholder={placeholder}></textarea>;
}

export default injectIntl(HelloWorld);

Hope this helped you.
Most information comes from @damonbauer's course.
Feel free to share your thoughts with me on twitter.