If you need to add another dictionary to check the content of your blog post to find spelling errors, you need to install another language pack. So you must go to the Windows Control Panel and select Clock, Language, and Region. Press Windows Live Language Setting and select the language of the new dictionary. When the installation is done the user interface (UI) of Windows Live Writer will change to the new language. You can revert this running the Windows Live Language Setting again and select the original language.