According to Wikipedia:

The animation for the film was created in the Madhouse Studios in Tokyo while the post-production work was done in California. The English soundtrack for the film was recorded in 1999 before the Japanese dialogue was finished.

and

The completed version of the film was only released theatrically in an English-language version. On its Japanese theatrical release, it was subtitled in Japanese.

I don’t know if this was done in an effort to appeal more to Western audiences and increase chances of an overseas box office success, or for artistic reasons (the setting is much more Western- than Eastern-influenced). Either way it was an interesting production choice.