In many high-level languages there is support for globalization. (E.g. this one is for C#: https://docs.microsoft.com/en-us/dotnet/standard/globalization-localization/)
However, C does not have this kind of support.
C++ does. So if you're not limited to the use of C, but can use C++ as well, read this: https://stackoverflow.com/questions/15220861/how-can-i-set-the-decimal-separator-to-be-a-comma
If you really have to be oldschool C, then you have to write your own function.
What I'd do is probably something like read two integral numbers separated by comma. Then the first number will be the integral part while the second will be the fractional part.
scanf("%d,%d", &integral, &fractional);
double number = fractional;
// TODO: Make sure the fractional part cannot be negative. Report an error if it is!
// TODO: Make sure invalid input is not accespted. E.g. a,bc
while (number > 1)
number /= 10.0;
number += integral;
printf("Enter a floating point number (e.g. 1,23): ");
double d = ReadDouble();
printf("\nYou entered: %lf\n", d);
Another alternative is that you read a whole line and start processing the line character by character.
Good luck! :)