From OSX Help:
You can change the language shown in menus and dialogs; see dates, times, and numbers displayed according to the conventions of a geographic region; and even write in a language that uses a different writing system (or script) from yours without having the keyboard thats designed for that language. You set these options using International preferences (choose Apple menu > System Preferences and click International).
If you set up your computer to write in more than one language or to type special characters (for example, mathematical symbols or arrows), the Input menu appears as a flag on the right side of the menu bar. When you want to type in another language or type special characters or symbols, you choose an item from the Input menu.
You can also view multilingual text documents on your computer. Applications that support Unicode, a worldwide standard for encoding multilingual text, can display any language. Other applications may not support all languages and you may see missing or strange characters in the text. If you have trouble viewing multilingual text, open the document using a Unicode application, such as TextEdit.
The applications that came with your computer let you write in any language that Mac OS X supports. However, not all applications let you change the language shown in menus and dialogs. To see what languages an application supports, select the application icon, then choose File > Get Info, and click the Languages triangle.
