Here is method that you can use to access and change the Ribbon Window text-alignment which is centered by default. Method access the reference to the Ribbon control that you placed on RibbonWindow.

C#:

private void ChangeRibbonTitleAlignment(Ribbon ribbonControl)
{
    RibbonContentPanel contentPanel = ribbonControl.Template.FindName("PART_RCP", ribbonControl) as RibbonContentPanel;
    if (contentPanel != null)
    {
        foreach (UIElement item in contentPanel.Children)
        {
            if (item is RibbonBackgroundChrome)
            {
                TitlePanel panel = ((RibbonBackgroundChrome)item).Template.FindName("PART_TitleContent", (FrameworkElement)item) as TitlePanel;
                if (panel != null)
                {
                    foreach (UIElement panelChild in panel.Children)
                    {
                        if (panelChild is Label && ((Label)panelChild).Name == "SysTitleLabel")
                        {
                            Label titleLabel = (Label)panelChild;
                            titleLabel.HorizontalContentAlignment = HorizontalAlignment.Left;
                            break;
                        }
                    }
                }
                break;
            }
        }
    }
}

VB:

Private Sub ChangeRibbonTitleAlignment(ByVal ribbonControl As Ribbon)
    Dim contentPanel As RibbonContentPanel = TryCast(ribbonControl.Template.FindName("PART_RCP", ribbonControl), RibbonContentPanel)
    If contentPanel IsNot Nothing Then
        For Each item As UIElement In contentPanel.Children
            If TypeOf item Is RibbonBackgroundChrome Then
                Dim panel As TitlePanel = TryCast(DirectCast(item, RibbonBackgroundChrome).Template.FindName("PART_TitleContent", DirectCast(item, FrameworkElement)), TitlePanel)
                If panel IsNot Nothing Then
                    For Each panelChild As UIElement In panel.Children
                        If TypeOf panelChild Is Label AndAlso DirectCast(panelChild, Label).Name = "SysTitleLabel" Then
                            Dim titleLabel As Label = DirectCast(panelChild, Label)
                            titleLabel.HorizontalContentAlignment = HorizontalAlignment.Left
                            Exit For
                        End If
                    Next
                End If
                Exit For
            End If
        Next
    End If
End Sub