private static final String FXML_MAIN = "fxml/main.fxml";
public static final String BUNDLES_FOLDER = "Locale";

private Stage primaryStage;

private MainController mainController;
private FXMLLoader fxmlLoader;

private VBox currentRoot;

@Override
public void start(Stage primaryStage) throws Exception {
    this.primaryStage = primaryStage;
    createGUI(LocaleManager.RU_LOCALE);
}

public static void main(String[] args) {
    launch(args);
}

@Override
public void update(Observable o, Object arg) {
    Lang lang = (Lang) arg;
    VBox newNode = loadFXML(lang.getLocale()); // получить новое дерево компонетов с нужной локалью
    currentRoot.getChildren().setAll(newNode.getChildren()); // заменить старые дочерник компонента на новые - с другой локалью
}

// загружает дерево компонентов и возвращает в виде VBox (корневой элемент в FXML)
private VBox loadFXML(Locale locale) {
    fxmlLoader = new FXMLLoader();
    fxmlLoader.setLocation(getClass().getResource(FXML_MAIN));
    fxmlLoader.setResources(ResourceBundle.getBundle(BUNDLES_FOLDER, locale));

    VBox node = null;

    try {
        node = (VBox) fxmlLoader.load();

        mainController = fxmlLoader.getController();
        mainController.addObserver(this);
        primaryStage.setTitle(fxmlLoader.getResources().getString("address_book"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    return node;
}

private void createGUI(Locale locale) {
    currentRoot = loadFXML(locale);
    Scene scene = new Scene(currentRoot, 300, 275);
    primaryStage.setScene(scene);
    primaryStage.setMinHeight(600);
    primaryStage.setMinWidth(400);
    primaryStage.show();
}}

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalStateException: Location is not set.
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2434)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2409)
    at start.Main.loadFXML(Main.java:56)
    at start.Main.createGUI(Main.java:68)
    at start.Main.start(Main.java:33)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    ... 1 more
Exception running application start.Main