One thing I’ve noticed on Mac OS X 10.5 Leopard is that even though you select “United Kingdom” for the keypad, locales etc, the spell checking still stays in American English. This is quite annoying, and not an obvious thing to change.. but it can be done, so that British English is the default.
In any text box in any application, if you click in the box then go to…
"Edit Menu: Spelling and Grammar: Show Spelling and Grammar"
… you will get a box which allows you to change the language in the bottom left. If you change this to your preferred language (in my case British English), and close it.. the setting will apply system wide (you may have to close and reopen any applications currently open to see the change).