I hate using devices in my native language, so I normally use the US locale. However, I also want to have Celcius, meters, kilograms and sane date formatting :)
I have a Wear watch and it uses the phone's locale to know what units to use. This means I get Fahrenheit weather and mile distances.
Is there a locale setting I can use to get SI units but retain English?
Try setting the locale to Canadian english (en_CA) which sets its units to metric. Things like dates and currency are fairly compatible with the US.
You could always set the dictionary for spell check to US english under Languages & Input in settings. Though, as you point out in your comment, you may have to deal with some "misspellings" in some menus and messages. It will add some colour to your android experience.
Q & A