@Component(immediate = true)
public class MainApplicationService implements IMainApplicationService {
    private MainStage mainStage;

    @Activate
    void activate() {
        BundleContext bundleContext = FrameworkUtil.getBundle(getClass()).getBundleContext();
        IEclipseContext osgiContext = EclipseContextFactory.getServiceContext(bundleContext);
        ContextInjectionFactory.setDefault(osgiContext);

        this.mainStage = ContextInjectionFactory.make(MainStage.class, osgiContext);
        this.mainStage.initComponents();
    }
}

@Creatable
public class MainStage {

        @Inject
        private NamedMessageRegistry registry;

        private void initComponents() {
                Label label1 = new Label();
                Label label2 = new Label();
                Label label3 = new Label();

                this.registry.register(label1::setText, m -> m.first_label_message);
                this.registry.register(label2::setText, m -> m.second_label_message);
                this.registry.register(label3::setText, m -> m.third_label_message);
        }
}

public class MainStage {

    // ...
    @Inject
    private IEclipseContext      context;

    //...

    private void initComponents() {

        //...
        Button btn = new Button("Button");
        btn.setOnAction(event -> {
            context.set(TranslationService.LOCALE, Locale.ENGLISH);
        });
    }
    //...

    @Inject
    @Optional
    private void getNotified(@Named(TranslationService.LOCALE) Locale s) {
        System.out.println("Injected via context: " + s);
    }
}