It's actually very simple. You just need to create your own resource directories for your language id strings and android will automatically pick the correct one.

Creating Local Directories and Resource Files To add support for more locales, create additional directories inside res /. Each directory name must adhere to the following format:

-b + [+] For example, values-b + es / contains string resources for locales with es code language. Likewise, mipmap-b + es + ES / contains icons for locales with es language code and ES country code. Android loads the appropriate resources according to the device's language settings at runtime. For more information, see the "Providing Alternative Resources" section.

After you have decided what locales will support, create a resource subdirectories and files. For example:

MyProject / res / values ​​/ strings.xml values-b + es / strings.xml mipmap / country_flag.png mipmap-b + es + es / country_flag.png For example, the following are some resource files for different languages:

English strings (default language), / values ​​/ strings.xml:

    Hello World! Spanish strings (es locale), / values-es / strings.xml:

    ¡Hola Mundo! United States flag icon (standard locale), / mipmap / country_flag.png:

United States flag icon Figure 2. Icon used for the default (en_US) locale

Spain flag icon (es_ES locale), / mipmap-b + es + ES / country_flag.png:

Spain flag icon Figure 3. Icon used for es_ES locale

Note. You can use the locale qualifier (or any qualifier configuration) for any type of resource, for example if you want to provide localized versions of your bitmap. For more information, see Localization.

Use resources in your application

As in Android developer page: https://developer.android.com/training/basics/supporting-devices/languages.html

If you don't want Android to automatically select the language of your assets, you can change the language programmatically (not recommended). But you need to change the language for each activity:

 @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);    
    String languageToLoad  = "fa"; // your language
    Locale locale = new Locale(languageToLoad); 
    Locale.setDefault(locale);
    Configuration config = new Configuration();
    config.locale = locale;
    getBaseContext().getResources().updateConfiguration(config, 
    getBaseContext().getResources().getDisplayMetrics());
//setContentView...
}

      

        
        
        
      

    

Or you install your language via adb if it is for testing only (however via settings it is probably close as fast as this solution):

adb shell "setprop persist.sys.language en; setprop persist.sys.country CA; setprop ctl.restart zygote"