Switching dictionaries is easy when you know how to do it. The only requirement is to use applications which use system spell checker (Apple applications and many third party Cocoa applications).

  1. Open an application that uses a built-in dictionary. Mail is an example.
  2. Choose “Spelling” from the “Edit” menu, and “Spelling…” from the submenu.
  3. In the “Spelling” dialog box, select the desired dictionary language from the pop-up menu.
  4. Close the window.

But remember: all language may not have an available dictionary, there are no English variants (only US English is available), and French dictionary is not the one with 1990 changes.

Source: Mac OS X 10.2: How to Change the Spell Check Language Dictionary