App.js

import React, {Component} from "react";

import intl from 'react-intl-universal';
// common locale data
require('intl/locale-data/jsonp/el.js');

export default class App extends Component {

  state = {initDone: false};

  componentDidMount() {
    this.loadLocales();
  }

  loadLocales() {
    intl.init({
      currentLocale: i18nConfig.locale,
      locales,
    })
      .then(() => {
        // After loading CLDR locale data, start to render
        this.setState({initDone: true});
      });
  }
...