It’s very convenient how all Pokemon understand human language. There are probably a few feral ones that don’t, but most do from the get go.

Not much to add here. This just came to mind.
Not sure if there's a reason for it. Or if one's even needed, as they are fictional creatures. But it's kinda random, yet very convenient.
Seeing how Pokemon are their world's equivalent of our animals, it's funny how the norm is them understanding human speech.