Will be the owner of a mac, but...

It is possible, but I think you'll get a list of languages to select from, and say you scroll over English, it will read out in the English voice. You must know that I am not certain of what I'm saying here, I'm just thinking this is what will happen. I live in the UK, and when I got my Mac new, it was set to English US and read out the other languages in the voice of that language if you know what I mean. So what I'm basically saying is the first language may be finish, but it should read out English US or UK or whatever you need when you scroll past it then read the rest of the set up in selected language.

I would just reinstall the whole OS X if I where you, if it isn't already done when you get the mac. Let's also hope it has Mountain Lion installed. Then you would need nothing else than the Mac it self. And this way you'll also get a new, fresh Mac, guaranteed clean for any traces from the previous owner. VO should also work through out the whole install process. You just have to manually toggle it on with cmd+F5. Here’s how to install (or reinstall) OS X, step by step: Boot from your Recovery HD partition by restarting your Mac while holding down the cmd+R keys. The OS X Utilities window appears. Select Reinstall OS X, and click Continue. The OS X Mountain Lion splash screen appears. Click Continue. A sheet appears informing you that your computer’s eligibility needs to be verified by Apple. Click Continue to begin the process of installing or reinstalling OS X. If you’re not connected to the Internet, you’ll be asked to choose a Wi-Fi network from the AirPort menu in the top-right corner. The Mountain Lion software license agreement screen appears. Read it and click Agree. A sheet drops down, asking whether you agree to the terms of the license agreement. Yes, you did just click Agree; this time you’re being asked to confirm that you indeed clicked the Agree button. If you don’t click Agree you can’t go any farther, so I advise you to click Agree now. Choose the disk on which you want to reinstall OS X by clicking its icon once in the pane where you select a disk. If only one suitable disk is available you won’t have to choose; it will be selected for you automatically. Click the Install button. A sheet asks for your Apple ID and password. Type them in the appropriate fields; click Sign In, and your Mountain Lion installation (or reinstallation) begins. The operating system takes 30 to 60 minutes to install, so now might be a good time to take a coffee break. When the install is finished, your Mac restarts itself. If you were reinstalling OS X on a hard disk that it had been installed on previously, you’re done. Your Mac will reboot and you can begin using your new, freshly installed (and ideally trouble-free) copy of OS X Mountain Lion. If you’re installing Mountain Lion on a hard disk for the first time, however, you still have one last step to complete. After your Mac reboots, the Setup Assistant window appears. You need to work your way through the Setup Assistant’s screens.