title: Jest testing: custom currency/decimal formatters showed wrong delimiters
body: <p>I tried to switch to Jest-testing inside of our React application.<br>
  Lots of mocha and enzyme tests are working with small fixes.</p>
  
  <p>With tests for a currency formatting component I get the wrong culture format inside the tests. I expect the result to be in format:  </p>
  
  <blockquote>
    <p>1.234.567,89 € </p>
  </blockquote>
  
  <p>but I receive:  </p>
  
  <blockquote>
    <p>1,234,567.89 €</p>
  </blockquote>
  
  <p>In the application I get all values formatted nicely in German formats as expected.</p>
  
  <p><strong>BTW:</strong><br>
  Inside of a helper method we use following code for internal formatting</p>
  
  <pre><code>value.toLocaleString(['de-DE'], {
      style: 'decimal',
      useGrouping: true,
      minimumFractionDigits: 2,
      maximumFractionDigits: 2
  })
  </code></pre>