In the story, Piro is quite fluent in Japanese. This comes from playing so many games and watching so much anime. The problem is, he sounds a bit like a young girl when he speaks (he talks with a lot of feminine manerisms). This is because he is mimiking what he hears in anime and games, which are mostly cute girls. He doesn't know he does this...
Largo doesnt speak a word of Japanese. You don't really need to speak any japanese to play the fighting games. He also has no interest in learning japanese.
Me? My japanese is so-so. I need a dictionary and a computer to really put anything together or translate. The Story Piro speaks and reads muuuuuch better than i do. His fluency is an exageration that helps with the story.