I’ve confirmed you found a bug in Xcode’s interface builder—it’s freezing off the number grouping separator at the time of the XIB creation, which it shouldn’t do if the ‘Localize’ box is checked. Please file a RADAR.
The following code works correctly, and ensures all localized properties (decimal point, group separator, etc) will get set at run-time:
self.textFieldCell.formatter = [NSNumberFormatter new];
((NSNumberFormatter *)self.textFieldCell.formatter).numberStyle = NSNumberFormatterDecimalStyle;