1. Changing the Default Dictionary
  2. Removing Learned Words From Your Dictionaries

Changing the Default Dictionary

As far as I've been able to determine there is no way to change the dictionary used for spell check system wide. This is annoying because you have to change it for each application, and each application is slightly different.

The below instructions are written for Mac 10.5.x (Leopard), the exact wording of menu's and steps may vary with other versions of the Mac.

Changing the Default Dictionary in Mail.app

Changing the Default Dictionary in Safari

One of the things I love about Safari is that it will spell check it text areas (recent versions of Firefox will as well). I don't believe that it's enabled by default so the first thing you have to do is enable it:

Now to change the dictionary:

Changing the Default Dictionary in Pages (iWork '08)

Removing Learned Words From Your Dictionaries

Ever accidentally added the wrong word to your system wide Mac dictionary? Wanted to see what all the words you've taught your Mac are? Or even decided that you want to erase all of the words you've taught your computer over the years?

I've tried a couple times to figure out how to do this and never managed, today I stumbled across the solution. You have one learned words file per dictionary that you have used, here you can see that I've used the standard English dictionary as well as the Australian English dictionary:

overkill(shand)$ ls ~/Library/Spelling/
en      en_AU

I've never found a "Mac way" of viewing the contents of these files, but it's very simple from the shell:

overkill(shand)$ strings ~/Library/Spelling/en
offlined
Fiona
maitred
Shand
captcha
Weta
Monique
adamshand
gmail
automount
renderwall
Pixar
commandline
anaglyphic

To remove all of these words from your dictionary all you have to do is delete the relevant dictionary file:

overkill(shand)$ rm ~/Library/Spelling/en

Hopefully this helps some other people!