How can I change the language for automatic spelling correction?

Question

I set my input source to be US English. And the flag showing on the taskbar is also an American flag.

But whenever I type something in Mail.app or Notes.app the spell check still corrects my spelling to British English:

• e.g. it highlights my "organize" and suggest s"organise".

Please, how to make my Mac (OS X 10.8) an American?

Answer 1

System Preferences → Languages & Text → Text → Spelling

Answer 2

To add to what gentmatt said, in the case you need to keep "Automatic by Language" because you happen to write in different languages, click on "Set Up…" in the list shown for the Spelling option.

On the dialog box that appears, you can change the order for the languages. In my case, this is the order I chose.

Now, I can write organize, and the word is not corrected, nor do I get a suggestion for writing organise. (It happens that organise is shown as incorrect.)