Anders Celsius wasn't an American. When I was in elementary school - 60 years ago - we used all the metric system measurements in our science classes in the certain knowledge that the US would soon convert from the British standard to metric. It has never happened. Metric is used almost exclusively in science and medicine - all my pills are in milligrams - but in the everyday world we hold onto the old standard. It's one of the few British things we really cling to. Despite all our technological advances, we haven't decided - our politicians haven't decided for us - to make the switch. I understand the British have switched as part of their EU membership. The US isn't the only western nation which uses Fahrenheit; there's Cayman Islands and Belize.
It's really no big deal; it's easy to convert from Fahrenheit to Celsius: [°C] = ([°F] − 32) × 5⁄9
Now that I think of it, Daniel Gabriel Fahrenheit wasn't American either.