You mean they can't?
I thought they, like, understood the attacks and stuff.
Think of it this way:
In real life, you can train dogs to perfrom actions such as 'sit' or 'rollover'.
Obviously, dogs can't understand human language but they can
familiarize a command according to the sound.
I'm guessing it's the same deal with Pokemon. However, there are faults to this theory.
It takes time to train a dog to perform actions; in Pokemon, however, they perform
the 'move' right off the bat with the first command of it's trainer. Another fault is that
Ash and Pikachu, along with many other trainers, can 'communicate' with each other
even though they obviously don't speak the same language.