Fill the TreeView with sample data

MainPage.xaml.cs

public sealed partial class MainPage : Page
{
    public MainPage() => InitializeComponent();

    private void OnSelectionChanged(TreeView sender, TreeViewItemInvokedEventArgs args)
    {
        if (args.InvokedItem is TreeViewNode node && node.Content is CultureData cd)
        {
            ViewModel.SelectedCulture = cd;                
        }
    }

    public CulturesViewModel ViewModel { get; } = new CulturesViewModel();

    protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        void AddSubNodes(TreeViewNode parent)
        {
            if (parent.Content is CultureData cd && cd.SubCultures != null)
            {
                foreach (var culture in cd.SubCultures)
                {
                    var node = new TreeViewNode
                    {
                        Content = culture
                    };
                    parent.Children.Add(node);
                    
                    foreach (var subCulture in culture.SubCultures)
                    {
                        AddSubNodes(node);
                    }
                }
            }
        }

        base.OnNavigatedTo(e);
        
        var rootNodes = ViewModel.RootCultures.Select(cd => new TreeViewNode
        {
            Content = cd
        });

        foreach (var node in rootNodes)
        {
            treeView1.RootNodes.Add(node);
            AddSubNodes(node);
        }
    }
}