title: How to define localization with defineMessages method?
body: <p>I want to localize my component. I use yahoo/react-intl lib. I define some messages</p>
  
  <pre><code>const messages = defineMessages({
      greeting: {
          id: 'app.home.greeting',
          description: 'Message to greet the user.',
          defaultMessage: 'Hello!'
      },
    });
  </code></pre>
  
  <p>I don't know how to define a localizable text for message so I got this error</p>
  
  <pre><code>app.js:27785[React Intl] Missing message: "app.home.greeting" for locale: "nl", using default message as fallback.
  </code></pre>
  
  <p>Example of using</p>
  
  <pre><code>&lt;input type="text" {...firstName} placeholder={intl.formatMessage(messages.greeting)} /&gt;
  </code></pre>
  
  <p>Does anyone know how to define localized messages?
  It looks not possible to do this with defineMessages.
  Thanks in advance.</p>