At the present time VoiceOver in OS X includes built-in voices that speak over 30 languages! This article explains how to select the voices you’d like to have available to you, and how to switch from one voice to another while using VoiceOver.
To customise the languages which are available to you, open the VoiceOver Utility. If you are using VoiceOver you can press VO-F8 to do this, otherwise navigate to the Applications > Utilities folder and you will find the VoiceOver Utility application there.
Inside the VoiceOver utility, press command-F3 to navigate to the Speech category. Using VO-right arrow or your mouse, navigate to the default voice popup button. Open the popup button (VO-space) and navigate to the last item, which is “Customize…” and select it.
You will now be on the dialog box with a large table listing all the voices which are available in the system. Check the checkbox beside the voices which you want available, and uncheck those you don’t want available. When you’re done selecting, press enter to save your selections and close the dialog box.
Now you can switch voices at any time while using VoiceOver. Open the VoiceOver rotor with VO-command-left arrow (and VO-command-left/right arrow if necessary to the voice area of the rotor) you can use VO-command-up/down arrows to select any of the voices you set up in the previous table.
If you are new to using VoiceOver on your Mac, I highly recommend AppleVis’s Mac OS X guides which will teach you many useful skills.