Actually, that just shows how little you know about Pikachu.
It actually can speak in its own way, but it's more obvious in the Japanese version.
For example, it calls Kasumi (Misty) Pikachupi. The "ka-chu-pi" is how it renders "ka-su-mi." For the most recent example, see the episode near the end of Advanced Generation when Satoshi recieves the Kasumi lure.
Satoshi (Ash) is Pikapi. Pi-ka-pi is as close to "sa-to-shi" as Pikachu can get. You'll hear this one a LOT, especially if the two are seperated.
It's kind of insulting that you'd insist someone to keep quiet just because they don't speak your language. It's rude. I know these are just Pokemon, but would you say that to a human speaking something other than English to a friend who understands them?
Pipipi is how Pikachu says "to-ge-pi" and so on. You'll hear it when Togepi gets in trouble or goes missing and the crowd is searching for it. There are probably more, but those are the most obvious.
In short, Pokemon speak and close trainers understand them.
Also, Piplup is annoying, but Pochama's Japanese voice is hilarious. Much more boyish and less oddly highly-pitched. Give it a listen sometime, it's very cute.
