How else do you think he hangs around with his celeb friends all the time? Do you think his assistants are with him 24/7?
I always wondered about that. His first english language speeches occured during MGS4's development and he pretty much said he listened to a pre-recorded version of his prepared speech to memorize it phonetically. Either way, even if his english improved by now (Which it probably has to some extend.) doesn't really change the fact that he didn't speak much english during MGS1 and 2's development, where the Blaustein drama occured.