ChromeOS OOBE: Add ARIA-labels for OOBE screens.

chrome/browser/resources/chromeos/login/oobe_welcome_dialog.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 Polymer({
   is: 'oobe-welcome-dialog',
   properties: {
     /**
      * Currently selected system language (display name).
      */
     currentLanguage: {
       type: String,
       value: '',
     },
     /**
      * Controls visibility of "Timezone" button.
      */
     timezoneButtonVisible: {
       type: Boolean,
       value: false,
     },
+
+    languageButtonLabel_: {
+      type: String,
+      value: "languageButtonLabel",
+    },
   },
 
   onLanguageClicked_: function() {
     this.fire('language-button-clicked');
   },
 
   onAccessibilityClicked_: function() {
     this.fire('accessibility-button-clicked');
   },
 
   onTimezoneClicked_: function() {
     this.fire('timezone-button-clicked');
   },
 
   onNextClicked_: function() {
     this.fire('next-button-clicked');
-  }
+  },
+
+  /**
+    * This function formats message for labels.
+    * @param String label i18n string ID.
+    * @param String parameter i18n string parameter.
+    * @private
+    */
+  formatMessage_: function(label, parameter) {
+    return loadTimeData.getStringF(label, parameter);
+  },
 });