ModernCommands.cs

using System.Linq;
using System.Windows.Input;
using FirstFloor.ModernUI.Windows.Controls;
public static class ModernCommands
{
    static ModernCommands()
    {
        var nextBinding = new CommandBinding(ModernCommands.NextTab, ExecuteNextTab, CanExecuteNextTab);
        CommandManager.RegisterClassCommandBinding(typeof(ModernTab), nextBinding);
        var prevBinnding = new CommandBinding(ModernCommands.PreviousTab, ExecutePreviousTab, CanExecutePreviousTab);
        CommandManager.RegisterClassCommandBinding(typeof(ModernTab), prevBinnding);
    }

    private static readonly RoutedCommand Next = new RoutedCommand("NextTab", typeof(ModernCommands));

    private static readonly RoutedCommand Previous = new RoutedCommand("PreviousTab", typeof(ModernCommands));

    public static RoutedCommand NextTab
    {
        get
        {
            return Next;
        }
    }
    public static RoutedCommand PreviousTab
    {
        get
        {
            return Previous;
        }
    }
    private static void CanExecuteNextTab(object sender, CanExecuteRoutedEventArgs e)
    {
        var tab = sender as ModernTab;
        if (tab == null || tab.SelectedSource == null)
        {
            e.CanExecute = false;
            return;
        }
        e.CanExecute = IsInRange(CurrentIndex(tab) + 1, tab);
    }
    private static void ExecuteNextTab(object sender, ExecutedRoutedEventArgs e)
    {
        var tab = (ModernTab)sender;
        tab.SelectedSource = tab.Links[CurrentIndex(tab) + 1].Source;
    }
    private static void CanExecutePreviousTab(object sender, CanExecuteRoutedEventArgs e)
    {
        var tab = sender as ModernTab;
        if (tab == null || tab.SelectedSource == null)
        {
            e.CanExecute = false;
            return;
        }
        e.CanExecute = IsInRange(CurrentIndex(tab) - 1, tab);
    }
    private static void ExecutePreviousTab(object sender, ExecutedRoutedEventArgs e)
    {
        var tab = (ModernTab)sender;
        tab.SelectedSource = tab.Links[CurrentIndex(tab) - 1].Source;
    }
    private static int CurrentIndex(ModernTab tab)
    {
        return tab.Links.IndexOf(tab.Links.FirstOrDefault(x => x.Source == tab.SelectedSource));
    }
    private static bool IsInRange(int i, ModernTab tab)
    {
        return i < tab.Links.Count && i > -1;
    }
}

Sample.xaml

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <mui:ModernTab Layout="Tab" x:Name="Tab">
        <mui:ModernTab.Links>
            <mui:Link DisplayName="Language" Source="/View1.xaml" />
            <mui:Link DisplayName="Item 2" Source="/View2.xaml" />
        </mui:ModernTab.Links>
    </mui:ModernTab>
    <StackPanel Grid.Row="1" Orientation="Horizontal">
        <Button Content="Previous" 
            Command="commands:ModernCommands.PreviousTab"
            CommandTarget="{Binding ElementName=Tab}"/>
        
        <Button Content="Next" 
            Command="commands:ModernCommands.NextTab"
            CommandTarget="{Binding ElementName=Tab}"/>
    </StackPanel>

</Grid>