If you do a bit of exploring and talking to people, you will eventually encounter someone who says that Chatot can learn human words. Once you obtain one (after trading with a girl in an early town) and get it to about level 20, it will learn the move Chatter. As they said, it can indeed learn human words. Once it learns Chatter, go to your Pokemon menu and select "Chatot". It will show Chatter along with summary, switch, and etc. From here, press A and record what you want to say in the DS mic. Not only will Chatot say what you said whenever you use the move Chatter, but it will become its cry. Note: This is not permanent, so you can change its Chatter as often as desired. Also, the speech will be slightly distorted so it will not sound exactly as you said it. The recorded sound lasts for only one day, so you will have to re-record it.