You can use NSNumberFormatter to have a localized number in your field when it displays.
NSNumberFormatter * formatter = [[NSNumberFormatter alloc] init];
[formatter setNumberStyle:kCFNumberFormatterDecimalStyle];
[formatter setMaximumFractionDigits:1];
[formatter setLocale:[NSLocale currentLocale]];
NSString * myString = [formatter stringFromNumber:[NSNumber numberWithFloat:123.456]];
from https://stackoverflow.com/a/11504127/719866
You can use the formatter in your check to see if the value entered by the user is valid for that locale with the -numberFromString:
method.