How do I prevent iOS keyboard from using anything BUT English?

How can I prevent the use of anything but English keyboard (iOS) when iPhoneKeyboard.Open() is called? (prevent players from typing in their own language)

You can’t. It’s driven entirely by the user’s iOS settings. Besides which, how would setting the keyboard prevent users from typing in their own language? Accents aside, you can type in most European languages just fine with an English keyboard.

Thinking about it further, you actually might be able to, by writing a native plugin which created its own input handler (those can override the user’s language settings). You’d need Unity iOS Pro for that, of course.

Again, though, it won’t stop anyone writing in Volapuk.