JDK: jdk 11 build 20
Env: Mac10.11, 10.12, 10.13
Setting locale steps:
Open Languages&Region, preferred languages select "Chinese, Simplified", region select "China".

Run following test on Mac 10.11, 10.12, 10.13 with zh_CN locale, but got different results.
public class Test4 {
    public static void main(String[] args){
        System.out.println(Locale.getDefault());
        System.out.println("Format locale: " + Locale.getDefault(Locale.Category.FORMAT));
        System.out.println("Display locale: " + Locale.getDefault(Locale.Category.DISPLAY));
    }
}

Output:
Mac 10.11:
zh_CN_#Hans
Format locale: zh_CN_#Hans
Display locale: zh_CN_#Hans

Mac 10.12:
zh_CN_#Hans
Format locale: zh_CN_#Hans
Display locale: zh_CN_#Hans

Mac 10.13:
zh_CN_#Hans
Format locale: en_CN_#Hans
Display locale: zh_CN_#Hans

On Mac 10.13, Java format locale is "en_CN_#Hans". It seems like Java does not setting the default format locale correctly on mac10.13.

For the format locale of mac 10.11, open Language&Region-->Advanced, there is a format language selection, refer to attachment advanced_10.11.png.

But for mac 10.12 and 10.13, there is no format language selection, refer to attachment "advanced_10.13.png". It seems like mac OS uses the "Preferred languages" as the format language. Java format locale on mac 10.12 is "zh_CN_#Hans", but on 10.13 is "en_CN_#Hans".

On mac 10.13, input "locale" in the command line, the output please refer to "locale10.13.png".

I also test with JDK 10.0.1+10 with de_DE locale on mac 10.13, the format locale is "en_DE".