Answer:
Well that is not true we use both Celsius and Fahrenheit thermometer
it just depends on where you are from, If you are from Celsius dominated country you use Celsius to measure whereas if you are from Fahrenheit dominated country you use it to measure the body heat.
If you have seen money heist (a netflix show), Helsinki uses Celsius Thermometer to check the body temperature of Arturo BECAUSE in spain they use celsius thermometer.
HOPE MY ANSWER WAS HELPFUL xD