MVVM messaging example

A more professional solution is to let the view model publish events in different contexts e.g. DataSaved or error events like FileNotFound.
The view becomes the observer and converts those events to messages for the user.
You can move this responsibilities to an e.g. Messenger class. Also consider to use IDs that map to actual messages for such application events.
This pattern becomes even more powerful if those events are published and observed via an event aggregator. This decouples the view from view model: adding more event sources will not break the observer classes, since the aggregator allows anonymous event observation.

MessageId.cs

enum MessageId
{
  Default = 0,
  FileNotFound
}

EventAggregator.cs

class EventAggregator
{
  // Register for all events named e.g. "ApplicationEvent" and of type 'EventHandler<ApplicationEventArgs>'
  public void RegisterListener<TArgs>(string eventName, EventHandler<TArgs> callBack)
  {
    ...
  }

  public void RegisterPublisher(object eventSource, string eventName)
  {
    ...
  }
}

ApplicationEventArgs.cs
Superclass for all custom application events args

class ApplicationEventArgs : EventArgs
{
  public MessageId Id { get; }
  public ApplicationEventArgs(MessageId id) => this.Id = id;
}

View Model

MainViewModel.cs

class MainViewModel : IViewModel
{
  public event EventHandler<ApplicationEventArgs> ApplicationEvent;

  public MainViewModel(EventAggregator eventAggregator)
  {
    // Broadcast event using the event aggregator
    eventaggregator.RegisterPublisher(this, nameof(this.ApplicationEvent));
  }

  private void OnFileNotFound()
    => OnApplicationEvent(MessageId.FileNotFound);

  protected virtual void OnApplicationEvent(MessageId messageId)
    => this.ApplicationEvent?.Invoke(this, new ApplicationEventArgs(messageId));
}

View

Messenger.cs

class Messenger
{
  // Notify e.g. MainWindow and deliver the resolved message
  public event EventHandler<MessageReceivedEventArgs> MessageReceived;

  public Messenger(EventAggregator eventAggregator)
  {
    // Observe all ApplicationEvent events
    eventAggregator.RegisterListener(nameof(IViewModel.ApplicationEvent), OnApplicationEvent);
  }

  private void OnApplicationEvent(object sender, ApplicationEventArgs e)
  {
    // Read (localized) messages e.g. from a JSON that stores messages as key-value-pair
    Dictionary<int, string> messages = ReadJsonMessageSource();
    if (messages.TryGetValue((int)e.Id, out string message))
    {
      DispatchMessage(message);
    }
  }

  private void DispatchMessage(string messageText)
  {
    // Create a complex message object
    var message = new Message()
    {
      Text = messageText,
      Image = GetMessageImage()
      Category = ...
    }

    OnMessageReceived(message);
  }

  private void OnMessageReceived(Message message)
    => this.MessageReceived?.Invoke(this, new MessageReceivedEventArgs(message));
}

MainWindow.xaml.cs

partial class MainWindow : Window
{
  private Messenger Messenger { get; }

  public MainWindow()
  {
    InitializeComponent();

    this.Messenger = new Messenger();
    this.Messenger.MessageReceived += OnMessageReceived;
  }

  private void OnMessageReceived(object sender, MessageReceivedEventArgs e)
  {
    var messageBox = new MessageBox(e.Message);
    messageBox.ShowDialog();
  }
}