I'm not quite sure, if I have the correct answer to your question, but I searched the plist-files and found something about regional localization. If you still need to change the language, you might want to give it a try.
Don't forget to backup the file (Info.plist) you're going to modify before doing so!
Open the package contents of "The Sims 3.app" in Applications > The Sims 3. Open the "Info.plist" (preferably with a plist-Editor) and change the child "Localization native development region" into your desired region. Save the file and have a try.
As I wrote, I don't know, if it works. But it was the only thing I found that might change the language and looked similar to the things you have to change on a PC...
