Step 1: Open International pref pane in System Preferences

Step 2: On the Languages tab, click on “Edit List”

Your list of languages already in the list might be longer – e.g., it’ll usually have most of the European languages.

Step 3: Select the language(s) you wish to add and click OK.

Here I’ve chosen both Australian English and British English – generally speaking though, these are virtually identical and you only need the one you prefer.

Step 3: The newly selected languages should now be in the list. Order the list to your preference by dragging list items around.

As it says under “Edit List”, the changes will take effect next time you start the application (in the case of Finder, that’s obviously when you restart). Generally speaking, I’d say leave English (i.e., the American one) in there somewhere.

Now you’re done for most of the system stuff. Pages will now create new documents with the top language as the default. However, for existing documents you’ll need to do the following:

Select all text, and then (1) click on Inspector, (2) click on the text tab, (3) select “More” tab, and (4) set the language.