ADDITIONAL SYSTEM INFORMATION :
Generic, at least Windows and macOS

A DESCRIPTION OF THE PROBLEM :
21.0.5 changed the locale fallback mechanism to no longer respect user.region in https://github.com/corretto/corretto-21/commit/a3cd6d6b916f7d8c03cd4b59324a6da11df467eb. The change should have only affected that properties can't be changed at runtime, but it breaks the existing user.region fallback logic.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Running with -Duser.region=DE makes this test pass on 21.0.4, but fail on 21.0.5:

Depending on your setup, you can run with -Duser.region=US and use "." as a separator in the test instead.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
0,50000
ACTUAL -
0.50000

---------- BEGIN SOURCE ----------
import static org.assertj.core.api.Assertions.*;
import org.junit.jupiter.api.Test;

class LocaleTest {

   @Test
   void test() {
      assertThat(String.format("%.5f", 0.5f)).isEqualTo("0,50000");
   }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Adding -Duser.country.format=DE fixes this issue, but not sure whether other properties need to be set.

FREQUENCY : always