How do I put the image on the right side of the text in a UIButton?

Simplest solution:

iOS 10 & up, Swift:

button.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)
button.titleLabel?.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)
button.imageView?.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)

Before iOS 10, Swift/Obj-C:

button.transform = CGAffineTransformMakeScale(-1.0, 1.0);
button.titleLabel.transform = CGAffineTransformMakeScale(-1.0, 1.0);
button.imageView.transform = CGAffineTransformMakeScale(-1.0, 1.0);

iOS 9 & up, Swift: (Recommended)

button.semanticContentAttribute = .forceRightToLeft

Despite some of the suggested answers being very creative and extremely clever, the simplest solution is as follows:

button.semanticContentAttribute = UIApplication.shared
    .userInterfaceLayoutDirection == .rightToLeft ? .forceLeftToRight : .forceRightToLeft

As simple as that. As a bonus, the image will be at the left side in right-to-left locales.

EDIT: as the question has been asked a few times, this is iOS 9 +.