Really? When I scanned the Brain Age manual, I didn't even find any "options" choice in the main menu.
Remember, no matter what language your Nintendo DSi XL is set to, the apparent title of your game in the system's main menu won't change. Go to the "System Settings" option in your Nintendo DSi XL's main menu. (It's black, and has a wrench as its symbol.) Go to the rightmost page and select the bottom choice. There, make sure the language is set to English.