Hm, so the question is how can Pokemon understand humans but they can not speak the human's language (in the case English)? Well, I have two theories:
1.) Pokemon are able to comprehend everything a human says, but have no motivation to learn English
2.) Some Pokemon have the ability to speak the language, but don't have the evolved characteristics to do so.
So let's start on the first theory, with a pivotal point: Meowth. In the anime, Meowth is the only known Pokemon who is able to speak to both Pokemon and humans. How was he able to do so? Well if you watched the first season of the anime you will know that Meowth learned so through listening to a teacher and repeating phrases such as "Sally sells seashells by the seashore" until he had somehow learned the entire English language. Why would he go through all of this trouble? Simple, in episode 72 of the anime, "Meowth Goes West" we see a flashback of Meowth's past in that Meowth tried to become more human like to attract a girl Meowth, Meowsy. In order to do that he learned English. Although, Meowth learned English it came with a price. He only knows one attack: Scratch. He did not learn Pay Day because he had spent all of his time learning a new language. This is why ordinary Pokemon don't have the motivation to learn English. If they spent their time learning a new language then they couldn't get stronger. Most Pokemon, such as Alakazam and Slowpoke for example, have IQs exponentially higher than humans. However, if they spent their time learning a language then they could not focus on evolving, and learning their core moves to keep up with the rest of their species.
Now for the second theory. Each Pokemon species is made uniquely and in a different way. Although they can communicate with each other, how would they be able to talk to humans. Think of a Lickitung, would a Pokemon like that be able to pronounce most human words with such a long tongue? Not only would it be creepy, but impossible as well! Like animals, even if they could talk most Pokemon don't have the characteristics to be able to speak another language.