add Chinese translation

Author: peilingjiang

client/components/Nav.jsx

@@ -657,6 +657,16 @@ class Nav extends React.PureComponent {
                 Deutsch
               </button>
             </li>
+            <li className="nav__dropdown-item">
+              <button
+                onFocus={this.handleFocusForLang}
+                onBlur={this.handleBlur}
+                value="zh-CN"
+                onClick={(e) => this.handleLangSelection(e)}
+              >
+                简体中文
+              </button>
+            </li>
             <li className="nav__dropdown-item">
               <button
                 onFocus={this.handleFocusForLang}

client/i18n.js

@@ -1,10 +1,18 @@
 import i18n from 'i18next';
 import { initReactI18next } from 'react-i18next';
 import Backend from 'i18next-http-backend';
-import { enUS, es, ja, hi, ptBR, de } from 'date-fns/locale';
+import { enUS, es, ja, hi, ptBR, de, zhCN } from 'date-fns/locale';
 
 const fallbackLng = ['en-US'];
-const availableLanguages = ['en-US', 'es-419', 'ja', 'hi', 'pt-BR', 'de'];
+const availableLanguages = [
+  'en-US',
+  'es-419',
+  'ja',
+  'hi',
+  'pt-BR',
+  'de',
+  'zh-CN'
+];
 
 export function languageKeyToLabel(lang) {
   const languageMap = {
@@ -13,7 +21,8 @@ export function languageKeyToLabel(lang) {
     ja: '日本語',
     hi: 'हिन्दी',
     'pt-BR': 'Português',
-    de: 'Deutsch'
+    de: 'Deutsch',
+    'zh-CN': '简体中文'
   };
   return languageMap[lang];
 }
@@ -25,7 +34,8 @@ export function languageKeyToDateLocale(lang) {
     ja,
     hi,
     de,
-    'pt-BR': ptBR
+    'pt-BR': ptBR,
+    'zh-CN': zhCN
   };
   return languageMap[lang];
 }