Masuda and Ohmiri in an interview implied that Pokemon aren't very sapient.
The Pokedex seems to imply that the vast majority of Pokemon aren't intelligent since Gen I.
New Pokemon Snap shows that Pokemon, at least the ones in Snap, are pretty much just magical animals with some small amount of anthropomorphism mixed in.
The games imply that most Pokemon can understand a few words at most. The talking pokemon page in Bulbapedia mentions a honedge but it doesn't hold a conversation with you, it only says "weeeeeelcoooome" like it doesn't know how to speak it properly.
https://youtu.be/MyNoXbejP3c?t=855Earlier in that video is a Vanilluxe who also says "cheeeeez" so there's implication that at best Pokemon can only pick up on a few words. Even Organguru speaks like a caveman and in short sentences and he's one of the most intelligent Pokemon.
Any showing of understanding may be attributed to what tvtropes calls "Amplified Animal Aptitude" which is when animals are shown to be more intelligent than what they really are for the sake of the plot. Lassie is a good example who can not only understand humans but also talk to them in some sort of dog morse code.
