I am seeing an issue where after I set a DocumentLanguage on the EditControl my TwoWay binding no longer works. If I simply remove the DocumentLanauge setting it works fine again. Here is the related XAML:
<syncfusion:EditControl Background="White" BorderBrush="Black" BorderThickness="1" Foreground="Black" Height="439" HorizontalAlignment="Left" Margin="0" Name="editControl1" VerticalAlignment="Top" Width="404" DocumentLanguage="SQL" Text="{Binding Path=MyStringProperty, Mode=TwoWay}"/>

I have tried using SQL and C# as Document languages with the same result.

Hi Phil,

Sorry for the inconvenience caused.

We have analyzed the reported issue with our sample; we are not able to reproduce the issue. Can you please check with the sample in the following location? If you still face any problem, please revert us by modifying the sample based on your application along with replication procedure. This would be helpful for us to serve you.

Sample : http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=Sample484142229.zip

Please let us know if you have any queries.

Thanks,
Karthick