When running the testsuite I get this failure:
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Test set: org.jboss.as.test.integration.jpa.beanvalidation.cdi.BeanValidationCdiIntegrationTestCase
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Tests run: 2, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 1.264 sec <<< FAILURE! - in org.jboss.as.test.integration.jpa.beanvalidation.cdi.BeanValidationCdiIntegrationTestCase
testFailingBeanValidation(org.jboss.as.test.integration.jpa.beanvalidation.cdi.BeanValidationCdiIntegrationTestCase) Time elapsed: 0.437 sec <<< FAILURE!
java.lang.AssertionError: expected:<[Not enough people for a reservation, may not be null]> but was:<[Not enough people for a reservation, darf nicht null sein]>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:743)
at org.junit.Assert.assertEquals(Assert.java:118)
at org.junit.Assert.assertEquals(Assert.java:144)
at org.jboss.as.test.integration.jpa.beanvalidation.cdi.BeanValidationCdiIntegrationTestCase.testFailingBeanValidation(BeanValidationCdiIntegrationTestCase.java:107)

Cause is, that the second part of the message is coming from the @NotNull contraint in Reservation.java and is in German, because it's a German Windows system.
Adding the expected English message to the NotNull constraint should solve this problem. I'll try and send a pull request later.