Bean Validation uses (wrong) default locale

Originally by DimitriL

Given is the code (also available in the attachment as a project):

First appearance of the fields shows the validation errors in the German language (see the screen shot) but the locale is set to UK. The reason is in the method of the AbstractComponent#getLocale() which returns null as the locale because the field is not yet attached to the parent, but the validator is already initialized with the default (wrong) locale.

public class ValidationUI extends UI {

	@Override
	protected void init(VaadinRequest request) {
		VaadinSession.getCurrent().setLocale(Locale.UK);
		setLocale(Locale.UK);

		final VerticalLayout layout = new VerticalLayout();
		layout.setMargin(true);
		setContent(layout);
		
		Project project = new Project();
		
		final TextField textField = new TextField("Name");
		textField.setNullRepresentation("");
		textField.setWidth("150px");
		
		final BeanFieldGroup<Project> fieldGroup = new BeanFieldGroup<Project>(Project.class);
		fieldGroup.setItemDataSource(project);
		fieldGroup.bind(textField, "name");
		
		FormLayout formLayout = new FormLayout(textField);
		formLayout.addComponent(textField);
		
		Button button = new Button("Click Me");
		button.addClickListener(new Button.ClickListener() {
			public void buttonClick(ClickEvent event) {
				try {
					layout.addComponent(new Label("Field Locale: " + textField.getLocale()));
					fieldGroup.commit();
				} catch (CommitException e) {
					throw new RuntimeException(e);
				}
			}
		});
		
		layout.addComponent(formLayout);
		layout.addComponent(button);
	}
}

Imported from https://dev.vaadin.com/ issue #12016

Reproduce the behavior with the ValidationUI example and inspect AbstractComponent#getLocale together with BeanFieldGroup initialization. Trace when the field is attached relative to validator setup; done means the first validation messages use the UI or session Locale.UK rather than the system default.