Spell checker for multilingual application

I am using using spellchecker in a multilingual application. Now I want to enable it for arabic culture. I am working on windows 7 machine and .net 3.5. As per msdn blogs, we don't need to install any language pack in my case. But still spell checker is not working. I have set the xml:Lang="ar" in my application. Please suggest.

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys1="clr-namespace:System;assembly=system"
        x:Name="MainScreen"
        Title="MainWindow" 
        xml:lang="ar"
        Height="350" 
        Width="525">
    <Grid><TextBox BorderBrush="Black" 
                 x:Name="EditableTextBox"
                 Background="Wheat" 
                 SpellCheck.IsEnabled="True"
                 BorderThickness="1" 
                 AcceptsReturn="True"
                 Height="40" Margin="25,160,37,111">
        </TextBox>
        
    </Grid>