Files to look at:
SpellChecker - An example of use
This example illustrates how to load multiple Dictionaries to check spelling of a multi-language text in a simple text box control. Also, you will see how to add a Custom Dictionary so that the end-user can add custom words to it.
Make sure that the "Copy to Output Directory" option for the dictionary files is set to "If Newer". This action prevents custom dictionary files from being overwritten.