During their trek to Olivine City, Ash notices Pikachu seems tired for some reason. Just then a man named Simon, who can communicate with Pokémon, offers his help. A short while later the local Officer Jenny claims that Simon is a fraud who has been scamming people. Is she right? And if not, who are the real culprits?
Simon says he speaks "fluent Pokemon", suggesting that although each Pokémon only says it's name (or makes other noises, such as Onix) this is the same language regardless of Pokémon species. There is no "Pikachu language" or "Bulbasaur language" etc., just one overall Pokémon language. This has been demonstrated on numerous occasions when different Pokémon easily communicate with each other as though they were speaking.
Officer Jenny says that Pokémon can't understand human speech, but this is obviously not the case, otherwise trainers would not be able to give their Pokémon orders in battle. Pikachu for example, has never had any trouble understanding Ash or any other humans in or out of battle, plus PIkachu has always been able to convey various messages to Ash, mainly through gesture/mime, but should have been able to defend Simon.