Well, the standard temperature unit is Celsius, so Fahrenheit is rare and used just by a few obscure countries: https://en.wikipedia.org/wiki/Fahrenheit#Usage -- and USA.

You are more likely to be in a country that has banned anti-personnel land mines than one using Fahrenheit.

The code does check your browser locale of "en-US" and sets it to the "wacky" unit (Fahrenheit). Given that it's plain vanilla JS and only some rudimentary XMLHttpRequest, I'm not sure it can do much more?