Difference between NSLayoutAttributeLeft vs NSLayoutAttributeLeading
"Leading" does not always mean "Left". For RTL-written languages (locales) leading edge of the object’s alignment rectangle will be located at the right side of the object.
Quote from Auto Layout Guide:
The attributes leading and trailing are the same as left and right for left-to-right languages such as English, but in a right-to-left environment such as Hebrew or Arabic, leading and trailing are the same as right and left. When you create constraints, leading and trailing are the default values. You should usually use leading and trailing to make sure your interface is laid out appropriately in all languages unless you’re making constraints that should remain the same regardless of language (such as the order of master and detail panes in a split view).