ADDITIONAL SYSTEM INFORMATION :
Windows 10 vs 1903
java version "13.0.1" 2019-10-15
Java(TM) SE Runtime Environment (build 13.0.1+9)
Java HotSpot(TM) 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)



A DESCRIPTION OF THE PROBLEM :
Whatever the operating system's language or an explicitly set default locale, JFileChooser is always displayed in English.
Was working correctly under jdk 10.0.2.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
(Uncomment line 8,) compile and run the code below.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Labels and buttons according to current default locale.
ACTUAL -
English

---------- BEGIN SOURCE ----------
import java.util.*;
import javax.swing.*;

public class FileChooserTest {

  public FileChooserTest() {
// If your operating system is English, set to any non-English locale.
// Locale.setDefault(Locale.FRENCH); // line 8
    System.out.println(Locale.getDefault());
    JFileChooser fileChooser= new JFileChooser();
    fileChooser.showDialog(null, null);
// fileChooser.showOpenDialog(null);
// fileChooser.showSaveDialog(null);
  }

  static public void main(String args[]) {
    SwingUtilities.invokeLater(FileChooserTest::new);
  }
}
---------- END SOURCE ----------

FREQUENCY : always