How can I get language of the android device in Ionic3, Cordova and Angular4 application?

You can use cordova-plugin-globalization which also offers an ionic-native wrapper. It offers a lot of useful methods, but you are probably looking for getPreferredLanguage() or getLocaleName().

Installation:

ionic cordova plugin add cordova-plugin-globalization
npm install --save @ionic-native/globalization

Example:

import { Globalization } from '@ionic-native/globalization';
constructor(private globalization: Globalization) { 
  this.globalization.getPreferredLanguage()
    .then(res => console.log(res))
    .catch(e => console.log(e));
}

window.navigator.language worked for me with no plugin. I didn't check to see how the results compare to the Globalization plugin, but I got results like: Android: "en-us", "es-us" and on iOS: "en-US" and "es-XL"

also see https://stackoverflow.com/a/4079798/431296 -window.navigator.userLanguage wasn't defined on either Android or iOS, so I didn't include it

Follow up to David's answer, the Globalization API is deprecated.

With the ECMA Internationalization API now supported on iOS, Android and Windows devices, this plugin is not required any more. * Migrating from this plugin to the ECMA Internationalization API is explained in this Cordova blog post.