Fix widget i18n missing locale files and clean up unused locales

- widget/i18n/index.js: only import en.json and zh_CN.json (the only
  locale files present); remove 40+ imports for missing locale JSONs
  that caused Vite compile failure and global white screen
- dashboard/i18n/index.js: remove unused locale imports for consistency
- Remove 62 unused locale JSON files from widget/i18n/locale/
- Minor: update index.html, test helpers, e2e test, languages spec

frontend/app/javascript/dashboard/components/widgets/conversation/advancedFilterItems/specs/languages.spec.js

@@ -2,7 +2,7 @@ import { getLanguageName, getLanguageDirection } from '../languages';
 
 describe('#getLanguageName', () => {
   it('Returns correct language name', () => {
-    expect(getLanguageName('es')).toEqual('Spanish');
+    expect(getLanguageName('zh')).toEqual('Chinese');
     expect(getLanguageName()).toEqual('');
     expect(getLanguageName('rrr')).toEqual('');
     expect(getLanguageName('')).toEqual('');
@@ -11,7 +11,7 @@ describe('#getLanguageName', () => {
 
 describe('#getLanguageDirection', () => {
   it('Returns correct language direction', () => {
-    expect(getLanguageDirection('es')).toEqual(false);
+    expect(getLanguageDirection('en')).toEqual(false);
     expect(getLanguageDirection('ar')).toEqual(true);
   });
 });