You can use multiple languages on your mac, check out the Podcast for details and potential caveats. Download it Here
First, go to System Preferences, and open International.
Now, 'Drag and drop' the languages to your preferred order. Localize it under the 'Formats' tab.
If you want to use the keyboard to type in the selected language, set that up as well under the 'Input Menu' You can localize user accounts, this is useful if you have bilingual people using your Mac.