raymcfetridge: To run a terminal command;
Go to your Applications folder and find the Utilities folder.
Open the Utilities folder and look for Terminal.app
Open up Terminal.app and a text only interface will appear. While decidedly less friendly than a graphical user interface the command line gives quite a bit of power to those who are looking for it.
Simply type in the command (or copy and paste what is below) and then press return. Once done just go ahead and quit Terminal.app.
defaults write-g NSPreferredSpellServerLanguage "en_CA"
For the curious the break down of the command:
defaults - an application that allows you to change preferences in programs, the operating system, etc.
write - Tells defaults that we are going to be adding information
-g - This is what is called a flag - the hyphen than a g means this is to be applied globally (meaning to every program installed)
NSPreferredSpellServerLanguage - This is a setting for programs allowing them to see what language they should be using for spellchecking. It's something you'll never see unless you're diving down into setting preferences manually like we are doing now.
"en_CA" - The Canadian English dictionary.
Hope that clears the mud slightly.