How to use a coma as the decimal separator?
Print the integer part, print a comma, then print the value minus the integer part.

float value = 123.45;
int ival = (int)value;
int frac = (value - ival) * 100;

Serial.print(ival);
Serial.print(",");
if (frac &lt; 10) Serial.print("0");
Serial.println(frac);