WPF MVVM Binding to static entity not updating

Question

I have a base abstract viewmodel containing 2 static models. In a usercontrol, I bind to both. Only the one binding (language) resolves and updates when the underlying static model changes. The binding to Employee does not update the fields. I have tested that the Employee setter is called and indeed it is. The getter however does not get called after the value changes. Any guidance will be appreciated.

Base Viewmodel:

namespace POC.Windows
{
  public abstract class ViewModel : ObservableObject, IDataErrorInfo
  {
    private static int _languageId;
    private static Languages _language;
    private static Employee _employee;

    public Employee Employee
    {
        get { return _employee; }
        set
        {
            _employee = value;
            NotifyPropertyChanged();
        }
    }

    public int LanguageId
    {
        get{return _languageId;}
        set{
            if (_languageId != value)
            {
                _languageId = value;
                LoadLanguage(); //Async populate Language model
                NotifyPropertyChanged();
            }
        }
    }

    public Languages Language
    {
        get
        {
            if (_language == null)
            {
                _language = new Languages();
                _languageId = -1;
                LoadLanguage();

            }
            return _language;
        }
        set
        {
            _language = value;
            NotifyPropertyChanged();
        }
    }
    //Async Loading functions here - omitted
  }
}

UserControl View:

<UserControl x:Class="POC.DesktopClient.UserControls.EmployeeDetails"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:viewModels="clr-namespace:POC.DesktopClient.ViewModels"
         mc:Ignorable="d"
         d:DataContext="{d:DesignInstance viewModels:EmployeeDetailsViewModel}">
<UserControl.Resources>
    <viewModels:EmployeeDetailsViewModel x:Key="EmployeeDetailsViewModel"/>
</UserControl.Resources>
<Grid>
 <StackPanel Orientation="Vertical" Grid.Row="0" Grid.Column="0">
   <TextBlock VerticalAlignment="Top" Margin="10" 
              Text="{Binding Path=Language.FirstName}"
              Foreground="DarkBlue" FontWeight="Bold" FontSize="20">
   </TextBlock>
 </StackPanel>
 <StackPanel Orientation="Vertical" Grid.Row="0" Grid.Column="1">
   <TextBlock VerticalAlignment="Top" Margin="10" 
              Text="{Binding Path=Employee.FirstName}"
              Foreground="DarkBlue" FontWeight="Bold" FontSize="20">
   </TextBlock>
 </StackPanel>
</Grid>

haven't touched the usercontrol's xaml.cs and EmployeeDetailsViewModel is empty:

public class EmployeeDetailsViewModel : ViewModel 
{ 
}

---EDIT--- Observable Object:

namespace POC.Windows
{
  public class ObservableObject : INotifyPropertyChanged
  {
    public event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null) 
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
  }
}

Additional information:

• The Usercontrol is hosted in the MainWindow.xaml.
• In MainWindow.xaml.cs a datacontext is defined as a MainViewModel (that also is blank and inherits from ViewModel).
• The static Employee model is bound to a listbox's selectedItem property.
• The listbox is in a separate usercontrol also on the MainWindow.
• Ths listbox's viewmodel is also blank and inherits from the base viewmodel. Its xaml.cs is blank, datacontext is set in Xaml, same as the details usercontrol.
• LanguageID is bound to a combobox selectedValue on the MainWindow.

So in a nutshell, when I change the language in the combobox, the usercontrol's labels bound to language updates correctly. When I select an employee in the listbox, usercontrol's labels bound to Employee remains blank.