The polyfill service applies the pollyfill to only an user agent which doesn't have the Intl feature implementation. This means the polyfill service doesn't apply the polyfill if you're using Chrome since Chrome supports the Intl features.

So, I've verified your result 2018 M12 17, Mon on Chrome. It's a behavior of Chrome not Intl pollyfill. I also found an issue related to hy locale for Chromium ( #842524 ). It might be related to your problem.

As a workaround for your problem, please try following step. You may be able to resolve your issue.

1. install the Intl pollyfill via npm

$ npm install intl

2. create a test.html file like this

<script src="./node_modules/intl/dist/Intl.js"></script>
<script src="./node_modules/intl/locale-data/jsonp/hy.js"></script>
<script>
var x = new IntlPolyfill.DateTimeFormat('hy', {
  weekday: 'long',
  year: 'numeric',
  month: 'long',
  day: 'numeric'
}).format(new Date);

console.log(x);
</script>

3. open the test.html and try to check the console on your browser

Thank you very much!