Add TextEncoder polyfill for test run

test/setupTests.js

@@ -1,6 +1,12 @@
 import * as languageHandler from "../src/languageHandler";
+import { TextEncoder, TextDecoder } from 'util';
 
 languageHandler.setLanguage('en');
 languageHandler.setMissingEntryGenerator(key => key.split("|", 2)[1]);
 
 require('jest-fetch-mock').enableMocks();
+
+// polyfilling TextEncoder as it is not available on JSDOM
+// view https://github.com/facebook/jest/issues/9983
+global.TextEncoder = TextEncoder;
+global.TextDecoder = TextDecoder;