To start the Java runtime with a different language, just set the user.language property,
as in java -Duser.language=FR ClassName.
Then, whenever you create a JOptionPane,
you would get the French labels for Yes, No, OK, and Cancel.
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.UIManager;
public class ConfigOptionPaneLanguage {
public static void main(final String[] args) {
//
UIManager.put("OptionPane.cancelButtonText", "Annuler");
UIManager.put("OptionPane.noButtonText", "Non");
UIManager.put("OptionPane.okButtonText", "D'accord");
UIManager.put("OptionPane.yesButtonText", "Oui");
JFrame parent = new JFrame();
String multiLineMsg[] = { "Hello,", "World"} ;
JOptionPane.showMessageDialog(parent, multiLineMsg);
}
}