Read original post by Sumit Maitra at .NET Curry
The WinRT runtime provides spell check and auto complete functions in controls like the TextBox and the RichTextBox out of the box. To do so, it hooks into a Windows 8 default system Dictionary that’s installed based on the language you choose for your Windows 8 setup. This provides red squiggles under incorrectly spelt words and provides a right click context menu with suggestions for fixing the error. This also provides an ‘Add to Dictionary’ functionality that adds new words to the Dictionary. However, there is no way to hook a custom dictionary into this setup. To enable this, all you have to do is set two properties on the XAML Control [...]