If you're just trying to map 0 -> the locale's version of "Sun", you can use -[NSDateFormatter shortWeekdaySymbols]
.
The documentation doesn't have any description of the ordering, but I've tried with en_US/he_IL for Sunday first and en_UK/fr_FR for Monday first. In all cases the array returns the locale equivalent of @[@"Sun", @"Mon", ...]
with Sunday being the first day.
Of course, your code may become invalid if you assume the locale is using a gregorian calendar, but that's really outside the scope of the question as that probably relates more to the design of the app as a whole.