Ah, gotta love the conceits of old Westerns and sci-fi!

It isn't JUST that the Indians speak fluent English, but that they even converse with one another in English! Sometimes plotpoints even involve the hero overhearing malign conversations among "Indians on the warpath."

We could assume that film narrative "translates" foreign languages into English for the convenience of the viewer, but a lot of the time the whole tribe really IS speaking among one another in English, because even the secondary white characters understand what's being said when, in YELLOWSTONE KELLY's case, even Kelly's young sidekick knows what's being said among his Sioux captors.

Same thing in old science fiction movies and TV shows (including Star Trek TOS and Lost in Space) -- the aliens all speak in fluent English toward Earthlings and even among one another.

Funny how I never thought about stuff like that, at the time. Most people probably didn't, either -- they just bought their movie tickets or tuned in to the TV station to be entertained, and checked cerebral nitpicking at the door in so doing.