Even non-Pokemon fans know that Pokemon are only capable of saying their own names. It's kind of their whole thing, in addition to being powerful, destructive monsters. That's why a choice in the most recent Pokemon movie, Pokemon The Movie: I Choose You!, was so upsetting to many of the fans in the audience.
Towards the end of the film both Ash and Pikachu are hurt after a vicious Pokemon attack. Ash asks Pikachu why he wouldn't go inside his Pokeball and Pikachu responds in English.