[MrSeanKon]

It is better to open the code in Visual Studio (when you have free time) and you will understand easily what is going mystic
But let me to explain you right now.
I define a groupbox which has a header and two buttons inside (button_EN & button_GR). Pressing a button translates the controls of WPF form from English to Greek and vice versa (buttonEN_Click & buttonGR_Click events).
If the main XMAL file is this:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Header's color and font change" Height="150" Width="300" ResizeMode="NoResize">
    <Grid>
        <GroupBox Name="grouplang" BorderThickness="3" Margin="20,10,30,20">
            <GroupBox.Header>
                <TextBlock FontSize="14" Foreground="Green" FontWeight="Bold" FontFamily="Times New Roman">Language</TextBlock>
            </GroupBox.Header>
        </GroupBox>
        <Button Content="Ελληνικά" Height="23" HorizontalAlignment="Left" Margin="50,50,0,0" Name="buttonGR" VerticalAlignment="Top" Width="75" Click="buttonGR_Click" />
        <Button Content="English" Height="23" HorizontalAlignment="Left" Margin="150,50,0,0" Name="buttonEN" VerticalAlignment="Top" Width="75" Click="buttonEN_Click" />
    </Grid>
</Window>

after pressing a button damn header's color & font's size/family changes....
Adding some definitions for TextBlock only the color changes (font family & size not).

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Here only the color changes" Height="150" Width="300" ResizeMode="NoResize">
    <Grid>
        <GroupBox Name="grouplang" BorderThickness="3" FontSize="14" Foreground="Green" FontWeight="Bold" FontFamily="Times New Roman" Margin="20,10,30,20">
            <GroupBox.Header>
                <TextBlock FontSize="14" Foreground="Green" FontWeight="Bold" FontFamily="Times New Roman">Language</TextBlock>
            </GroupBox.Header>
        </GroupBox>
        <Button Content="Ελληνικά" Height="23" HorizontalAlignment="Left" Margin="50,50,0,0" Name="buttonGR" VerticalAlignment="Top" Width="75" Click="buttonGR_Click" />
        <Button Content="English" Height="23" HorizontalAlignment="Left" Margin="150,50,0,0" Name="buttonEN" VerticalAlignment="Top" Width="75" Click="buttonEN_Click" />
    </Grid>
</Window>

Thus my question is how can I access groupbox's header color during execution adding the necessary commands in C#?
For WF case it is simple because WF design is based on C#.