
I noticed this while looking for old chats. I live in Korea but my primary language is English, and I noticed all the dates were coming out in U.K. style. Changing the language to English US or English Canada did not change the date form.
However, the other way around - US region, Korean language - shows the Korean date form (year, month, day).
My device is an iPhone 6S running iOS 10.0.1.