Google maps has a setting that allows you to select units of measure, it may default to what's common for your area, but it's worth a look. Oddly for Google assistant it looks like you can just tell it what you prefer. It would go something like this,
>OK, Google
>Use kilometers from now on.
>Okay, I'll remember that you prefer kilometers.
If you're having issue with Celsius you can change that by opening the Google app on your device, lower right hand corner, touch "More", (should be showing your Google account at the top) "Settings", Google Assistant, Weather (under the "You" tab). It's my understanding that you can change this the same as Kilometers, just by asking.