fix($theme-default): Make navbar dropdown links accessible

packages/docs/docs/theme/default-theme-config.md

@@ -72,6 +72,25 @@ module.exports = {
 }
 ```
 
+You can change `aria-label` of dropdown button to with a custom aria-label. Default value is text property. if the dropdown menu consists of language items., default value is 'Select language'
+
+```js
+module.exports = {
+  themeConfig: {
+    nav: [
+      {
+        text: 'Languages',
+        ariaLabel: 'Language Menu'
+        items: [
+          { text: 'Chinese', link: '/language/chinese/' },
+          { text: 'Japanese', link: '/language/japanese/' }
+        ]
+      }
+    ]
+  }
+}
+```
+
 You can also have sub groups inside a dropdown by having nested items:
 
 ```js

packages/docs/docs/zh/theme/default-theme-config.md

@@ -68,6 +68,24 @@ module.exports = {
 }
 ```
 
+
+```js
+module.exports = {
+  themeConfig: {
+    nav: [
+      {
+        text: 'Languages',
+        ariaLabel: 'Language Menu'
+        items: [
+          { text: 'Chinese', link: '/language/chinese/' },
+          { text: 'Japanese', link: '/language/japanese/' }
+        ]
+      }
+    ]
+  }
+}
+```
+
 此外，你还可以通过嵌套的 `items` 来在 `下拉列表` 中设置分组：
 
 ```js