Team Rocket's Meowth was once the only Pokémon with the ability to not only understand human speech but also converse with humans in their own tongue. Now, there is another Meowth -- one from the Mirror World -- that shares this exact same trait as its original.
A talking Meowth that is the mirror image of the original might not seem that revelatory, but this alternate Meowth is more than a clone. The Mirror World is a universe all of its own so the rules of clones don't apply in the same way that they do within the rest of the Pokémon franchise. That said, they do share some similar characteristics, such as the idea of destiny, which is what makes the idea of two talking Meowths more remarkable.
When they travel to the Mirror World, Ash, his traveling companions and Team Rocket all have identical Pokémon with the same moves as their real-world counterparts. Even though these two separate groups are in entirely different universes, they still travel together and form friendships and bonds -- meaning that while the universes may be in separate realities, the timelines are almost exactly the same. The only thing that distinguishes these universes from one another is that the Mirror World's inhabitants have the opposite traits to their real-world doppelgangers.
For instance, Team Rocket still formed a massive organization with ties all over the world -- the only difference is that they chose to stop crimes instead of actively committing them. There are small details that have changed across the two universes but the foundation of the main timeline story and the story of this universe are intertwined with one another. However, no matter what differences the characters have in personality and motivation, certain key elements essentially remain the same -- as demonstrated by Meowth and his Mirror World twin, who are both able to talk despite being from two different universes. These two Meowths could have learned to talk in different ways, but the fact that they will learn to communicate was always going to be the end result.
This actually boosts the mystique associated with Meowth; being such a common Pokémon that -- somehow -- is able to communicate with humans clearly and directly in the correct verbiage. It's no secret that Pokémon can understand human language, but it's all the more amazing when a Pokémon becomes able to mimic speech. Considering that this is something that has become set in stone across two universes now, it is quite the phenomenon.
Clearly, no matter where he is, Meowth's chatty destiny is something that's fixed within the Pokémon franchise.
