ISO Weeks with US Region Preferences

I want to use ISO week numbers (first week of year contains first Thursday) in my iOS calendar. My region preference is set to United States, and I don't want to change it. How do I have ISO week numbers with United States region preference?

I know I could switch my region preference to UK. (I assume this is because ISO weeks are more common in Europe than USA.) But this appears to also switch other preferences like how times and dates are formatted. I don't want to have UK time and date preferences, but I want ISO weeks.

I have an iPhone 6 running iOS 9.3.1.