set locale in number test (#4764)

Author: pjfanning

src/test/java/com/fasterxml/jackson/databind/ser/jdk/NumberSerTest.java

@@ -4,6 +4,8 @@
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.text.DecimalFormat;
+import java.text.DecimalFormatSymbols;
+import java.util.Locale;
 
 import org.junit.jupiter.api.Test;
 
@@ -98,7 +100,7 @@ public BigDecimal getValue() {
     }
 
     static class BigDecimalAsStringSerializer extends JsonSerializer<BigDecimal> {
-        private final DecimalFormat df = new DecimalFormat("0.0");
+        private final DecimalFormat df = createDecimalFormatForDefaultLocale("0.0");
 
         @Override
         public void serialize(BigDecimal value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
@@ -107,7 +109,7 @@ public void serialize(BigDecimal value, JsonGenerator gen, SerializerProvider se
     }
 
     static class BigDecimalAsNumberSerializer extends JsonSerializer<BigDecimal> {
-        private final DecimalFormat df = new DecimalFormat("0.0");
+        private final DecimalFormat df = createDecimalFormatForDefaultLocale("0.0");
 
         @Override
         public void serialize(BigDecimal value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
@@ -219,4 +221,9 @@ public void testCustomSerializationBigDecimalAsNumber() throws Exception {
         ObjectMapper mapper = jsonMapperBuilder().addModule(module).build();
         assertEquals(a2q("{'value':2.0}"), mapper.writeValueAsString(new BigDecimalHolder("2")));
     }
+
+    // default locale is en_US
+    private static DecimalFormat createDecimalFormatForDefaultLocale(final String pattern) {
+        return new DecimalFormat(pattern, new DecimalFormatSymbols(Locale.ENGLISH));
+    }
 }