Pokémon aren't normally able to speak directly to humans. Their dialogue is often limited to their own names and the occasional onomatopoeia. However, some Pokémon have managed to breach this speech barrier to let humans know exactly what they're trying to get across.
This isn't just about Meowth's ability to speak following extensive training, either. Pokémon Have found all sorts of ways for humans to hear and understand them. Here are some prime examples of such Pokémon that have managed to "speak" to humans.
Mewtwo and Other Legendary Pokémon Use Telepathy
Mewtwo doesn’t talk per se, but it can communicate in words that humans can understand. It does this by projecting its thoughts telepathically to anyone in range. Technically, Santa Clause's Lapras was the first Pokémon to use this form of communication, but Mewtwo was the one to popularize it. It became the main mode of “speech” for most Legendary and Mythical Pokémon in the movies, as well as a few other prominently featured Pokémon like Lucario and Zorua. That said, while it makes sense for a Psychic Pokémon like Mewtwo to communicate telepathically, the reason other types of Legendary Pokémon can do it is unclear.
Pokémon Like Beheeyem Can Speak Directly Through Meowth
This is a bit of a cheat, but a Pokémon can communicate with humans by using someone who already speaks the language, such as Meowth. Some of them can also take over a translator's body and "speak" through them. This method was originally used by a mutated Tentacruel, but Meowth has also been possessed by a Ghastly and Haunter, a Deoxys, and a group of Beheeyem. A Yamask could also communicate with humans by possessing Cilan. This mode of communication implies that all Pokémon have at least a passing understanding of human speech and are only limited by their own ability to speak it.
Ninetales and Zoroark Speak Through Human Illusions
Through the use of illusions, some Pokémon were able to mimic human speech. For example, in Episode 232, "Just Waiting on a Friend," a Ninetails conjured up the image of a girl named Lokoko, fooling Ash and his friends for a while. Even after the trick was discovered, the Ninetails continued to communicate with them using the illusion.
In Black and White Episode 127, "The Island of Illusions!" a similar situation appeared. In this case, a Zoroark was able to create a convincing illusion of Nurse Joy that fooled the main characters. In both cases, this demonstrates Pokémon's comprehension of both human behavior and human speech.
Manaphy Can Repeat a Limited Number of Words
Other Pokémon besides Meowth have learned human speech, which is unusual. A good example is Manaphy from Pokémon Ranger and the Temple of the Sea. It was quick to learn whatever lines were given to it even as a newborn Pokémon. It could even say full sentences by the end of the film, such as "I love you, mama."
Of course, Manaphy's dialogue is strictly limited in scope. Furthermore, because it has been released into the wild, it is unlikely to learn new dialogue anytime soon. Regardless, the fact that it learned anything at all is impressive.
Rotom Can Speak Through Certain Devices
When connected to the appropriate device, a Rotom can perfectly use human speech. This trend started with Pokémon the Series: Sun and Moon when Ash was constantly followed around by a Rotom Pokédex. Other Rotom were shown to be capable of human speech by entering phones and drones.
In these cases, the Rotom are more likely to say lines that are expected of the device they've possessed. If they have a Pokédex, for example, the majority of their conversation will revolve around Pokémon facts. Regardless, they are perfectly capable of using human speech.