Coming from a Polygon interview with Bill Trinen...

"They have these quirky voices, and you can adjust the voice for any character you want," he said. "You can adjust the pitch and the tone and things like that. So the first step was actually getting that to work well in English. Japanese being a phonetic language, it works very easily. English, it took a lot of work. Being able to hear them speak is a really key point of their personality, and hearing all the different voices and accents they have is also a very key part of how they come alive."

- development team in the U.S. has been working "over the last several years" with the Japanese development team