always run tests with locale en_US, no matter of system settings

build.gradle

@@ -98,9 +98,11 @@ tasks.withType(JavaCompile) {
     ]
 }
 
-// Use JUnit Jupiter
+// Configure tests
 tasks.named('test') {
     useJUnitPlatform()
+    jvmArgs '-Duser.language=en'
+    jvmArgs '-Duser.country=US'
 }
 
 // OpenAPI Source Code Generation

src/test/java/net/hostsharing/test/LocaleUnitTest.java

@@ -0,0 +1,17 @@
+package net.hostsharing.test;
+
+import org.junit.jupiter.api.Test;
+
+import java.math.BigDecimal;
+import java.util.Locale;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class LocaleUnitTest {
+
+    @Test
+    void testsAreUsingEnUsUtf8Locale() {
+        assertThat(Locale.getDefault()).isEqualTo(new Locale("en", "US"));
+        assertThat(new BigDecimal("64.00").toString()).isEqualTo("64.00");
+    }
+}