For the past 20 years, Pikachu has been voiced by Japanese voice actress Ikue Otani in both the Japanese and English versions of the anime, and when Pikachu speaks actual Japanese dialogue in the Japanese release of I Choose You!, it’s with Otani’s voice.
As foreign Pokémon fans have now heard, or seen for themselves, there’s a scene in I Choose You! where franchise mascot Pikachu speaks.
But on the other hand, when I Choose You! played in Japanese theaters, online reactions almost unanimously praised the scene where Pikachu speaks as the emotional high point of the entire film.