The plot revolves heavily upon promises made in person by President Abraham Lincoln to Chief Acoma just days before Lincoln's assassination. Lincoln never went to New Mexico (not even when it was a territory, as called in the film.)
Hans Conried, who is uncredited in this, said of his role, "Every tall actor wants to play Lincoln."
After being informed that Acoma speaks English, Abe Lincoln says "of course he speaks English, he's been here longer than the white man", implying America was an English-speaking country before the Europeans arrived.
There are a couple of comments about President Abraham Lincoln talking to Chief Acoma days before his assassination. The walking stick he presents to the Chief clearly has the date 1860 - Lincoln was assassinated in April, 1865, so the timeline between presentation of the gift and the assassination is not days, but is actually years. But it really doesn't matter, as ole Abe never made it to New Mexico as far as one knows.