When I kick following code with specifying user.language.display and
user.language.format, the output currency becomes "XXX".

public class decimalFormatTest {
  public static void main(String[] args) {
    DecimalFormat decif = new DecimalFormat();
    System.out.println(decif.getCurrency());
  }
}

% java -Duser.language.format=ja -Duser.language.display=fr decimalFormatTest
Currency = XXX

Then, if specify the user.country.format, output become correct one

% java -Duser.language.format=ja -Duser.language.display=fr -Duser.country.format=FR decimalFormatTest
Currency = EUR

But will not be correct when specify user.country.display only.

% java -Duser.language.format=ja -Duser.language.display=fr -Duser.country.display=FR decimalFormatTest
Currency = XXX

Then, if specifying user.language.display and user.country.display, seems that OS default is used.

% java -Duser.language.display=fr -Duser.country.display=FR decimalFormatTest
Currency = JPY


Also, NumberFormat.getCurrencyInstance() will work the same way as DecimalFormat.getCurrency().