You can also follow the steps listed in Order product | Platform, language swap - http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platform-language-swap.htm l to request a different language.
OK, here's the answer!
I finally got in touch with Adobe technical support by phone, since they couldn't help me on live chat. They told me to uninstall and reinstall to fix this. It would have been nice to simply change a setting instead, since it does take a good chunk of time to reinstall.
To uninstall on the Mac:
- Help>Deactivate (I used the Help menu in InDesign -- you only have to do it for one)
- Applications>Utilities>Adobe Installers>AdobeCS6 Design and Web Premium
- Uninstall all
Then I reinstalled using the disc. I noticed that during the reinstallation I couldn't see there are choices for types of English. The default is English (European), but I could only see the "English (...)" and I couldn't see that it was a pulldown menu and that the "English North American" was the second choice. It is the only thing special to do in the installation, yet I imagine there are others like me who are in the same boat.