I believe that Dogs can speak English, but their ability to do so is supernatural, so you can never discover it using science. For example, you can't dissect dogs to find out how this ability works. It is outside the scope of your ability to study or even understand.
Furthermore, dogs intentionally hide this ability from humans. Dogs only speak when humans aren't around, and they have the ability to know if humans are eavesdropping in any way (hence dogs have good hearing & can here things humans can't). So basically you will never catch them in the act.
You might want to know how I know this fact, well a cat told me. They're more talkative than dogs, but are in the same situation concerning speech, VonGlusenburg (talk to Von!) 00:39, May 27, 2012 (UTC).