That’s because it relies on the internalization API, which is not currently available in all browsers (for example not on iOS browser).
See the compatibility table.
This is a known issue (beta.1).
You can try to use a polyfill for Intl.
To do so, you can use the CDN version, and add this to your index.html:
<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=Intl.~locale.en"></script>
Or if you use Webpack, you can install the Intl module with NPM:
npm install --save intl
And add these imports to your app:
import 'intl';
import 'intl/locale-data/jsonp/en';