How do I use ResourceBundle for i18n in Java?

Creating an application for users in different regions can be hard in term of the message format for the local region. Java provide a ResourceBundle class that help internationalize our application.
To create resources for i18n (there are 18 letters between the first i and the final n) we need to create a file for each locale our application supported. The file name must be ended inlanguage_COUNTRY.properties. For instance a resource bundle for Locale.UK will be MessagesBundle_en_GB.properties.
When the bundle has been loaded we can use bundle.getString(key) to read specific message from our resource bundle file.

import java.util.Locale;
import java.util.ResourceBundle;

public class InternationalizationDemo {

    public static void main(String[] args) {
        //
        // Load resource bundle for Locale.UK locale. The resource 
        // bundle will load the MessagesBundle_en_GB.properties file.
        //
        ResourceBundle bundle = 
                ResourceBundle.getBundle("MessagesBundle", Locale.UK);
        System.out.println("Message in " + Locale.UK + ": " + 
                bundle.getString("greeting"));

        //
        // Change the default locale to Indonesian and get the default 
        // resource bundle for the current locale.
        //
        Locale.setDefault(new Locale("in", "ID"));
        bundle = ResourceBundle.getBundle("MessagesBundle");
        System.out.println("Message in " + Locale.getDefault() + ": " + 
                bundle.getString("greeting"));
    }
}

Below are some example of our resource bundle files, these files should be located in our application classpath to enable the ResourceBundle class to read it.

MessagesBundle_en_GB.properties
    greeting=Hello, how are you?

MessagesBundle_in_ID.properties
    greeting=Halo, apa kabar?