I just started playing around with Siri on my Watch Series 3. I live in northeast Thailand and asked Siri how far to Bangkok. The answer was in miles. I asked how many kilometers to Bangkok and the answer was in miles. I did just notice that I got the same answer from Siri on the iPhone until I changed the region setting, so that might be a clue. I get kilometers on every other app on the iPhone (Apple and Google Maps, Sygic). The temperature is in C (as I want it). I am associated with the US App Store, but that doesn’t seem to be a factor in any other app.
Thanks, Stephen. You were right. I had forgotten that there was a Language & Region setting in the watch app. Apparently I knew at some point because it was set to custom, which I don’t believe is the default setting. That accounts for why I made the change on my iPhone region setting, which corrected the units in the iPhone Siri response, but it didn’t get carried over to my watch. As soon as I checked Mirror my iPhone, the watch started giving me the distance in km.
Sometimes the most perplexing problems have the simplest solutions.