The Spell Checker plugin has two different spell checking modes. In the first mode it works with the native spelling dictionaries on each platform. On all versions of Mac OS X (that Xojo supports) and Windows 8 and above it uses the built-in spell checker dictionaries. If you can’t, or don’t want to, use the native dictionaries you can use the Hunspell dictionaries. There are hundreds of Hunspell dictionaries available for use in a variety of languages and speciality industries.
There is no Linux version at this point due to lack of interest.
More information, including downloadable demo, pricing, and usage is available at http://www.bkeeney.com/allproducts/bks-spell-checker-plugin-for-xojo/