In particular I want to guarantee that the formatting of a number uses , (commmas) for separating digits regardless of the current locale of the device.
NSNumberFormatter *numFormatter = [[NSNumberFormatter alloc] init];
[numFormatter setLocal:@"??????"]
The U.S. uses a period as a decimal separator. I assume you mean that you want to use a comma as a grouping separator.
You don't need to set the locale for this in your NSNumberFormatter, you can set both directly using code like the following:
[numFormatter setGroupingSeparator:","];
[numFormatter setUsesGroupingSeparator:YES];
[numFormatter setDecimalSeparator:"."];