A DESCRIPTION OF THE PROBLEM :
Before Java 11, a number of translations where included in the JRE and the JDK (e.g. Oracle JDK jdk-10.0.1).
They were removed from the compiled .jmod files since the introduction of Java 11 and the deprecation of the JRE.
Why?

It would be very helpful, if a java custom runtime image can include such translations.
I checked the source code and they still exist e.g. in case of plaf/basic:
JDK 11: http://hg.openjdk.java.net/jdk/jdk11/file/1ddf9a99e4ad/src/java.desktop/share/classes/com/sun/swing/internal/plaf/basic/resources

JDK 12: http://hg.openjdk.java.net/jdk/jdk12/file/1ddf9a99e4ad/src/java.desktop/share/classes/com/sun/swing/internal/plaf/basic/resources

I know, that there was a difference between the JRE and JDK until Java 6 but this is a long time ago:
 https://openjdk.java.net/groups/i18n/

REGRESSION : Last worked in version 11.0.1

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Setting the VM locale to -Duser.country=AT -Duser.language=de and open a YES/NO option dialog.

JFrame frame = ...
JOptionPane.showConfirmDialog(frame, "a message", "a title", JOptionPane.YES_NO_OPTION)


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
the buttons YES & NO should be translated to JA and NEIN
ACTUAL -
button label is YES & NO

FREQUENCY : always