Migrate spec/javascripts/locale/ to Jest

Closes gitlab-org/gitlab#194243

spec/javascripts/locale/index_spec.js → spec/frontend/locale/index_spec.js

 import { createDateTimeFormat, languageCode } from '~/locale';
 
-import { setLanguage } from '../helpers/locale_helper';
+import { setLanguage } from 'helpers/locale_helper';
 
 describe('locale', () => {
-  afterEach(() => {
-    setLanguage(null);
-  });
+  afterEach(() => setLanguage(null));
 
   describe('languageCode', () => {
     it('parses the lang attribute', () => {
@@ -22,14 +20,12 @@ describe('locale', () => {
   });
 
   describe('createDateTimeFormat', () => {
-    beforeEach(() => {
-      setLanguage('de');
-    });
+    beforeEach(() => setLanguage('en'));
 
     it('creates an instance of Intl.DateTimeFormat', () => {
       const dateFormat = createDateTimeFormat({ year: 'numeric', month: 'long', day: 'numeric' });
 
-      expect(dateFormat.format(new Date(2015, 6, 3))).toBe('3. Juli 2015');
+      expect(dateFormat.format(new Date(2015, 6, 3))).toBe('July 3, 2015');
     });
   });
 });