I'm kind of puzzled about Dio speaking Japanese. Isn't he British originally?

... Also for convenience's sake I'm assuming everyone just hears everything in the language they know, honestly.

Also, I could be misinterpreting posts, but Hermes isn't only audible to Kino. Anyone can hear his voice. He talks to other people multiple times in the anime.

... For some reason they don't ever seem too surprised by this.