Is there an English locale with SI units?
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 &amp; 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.