JAXB exception messages: How to change language?

OK, I found out that the language used for the JAXB event messages is determined by the java system property user.language. The default language is therefore dependent on the system it runs on.

I changed the run configuration of my java program by adding -Duser.language=en. This changed the language of the JAXB event messages to english.

You should use a validator to validate, not unmarshaller. You can change its locale like this:

import javax.xml.validation.Validator;import com.sun.org.apache.xerces.internal.impl.XMLErrorReporter;import org.xml.sax.ErrorHandler;Validator validator = schema.newValidator();ErrorHandler errorHandler = new YourErrorHandler();validator.setErrorHandler(errorHandler);XMLErrorReporter xmlErrorReporter = (XMLErrorReporter) validator        .getProperty("http://apache.org/xml/properties/internal/error-reporter");xmlErrorReporter.setLocale(new Locale("ru", "RU"));

Though with Java9+ you'd need to export the com.sun.org.apache.xerces.internal.impl package from the java.xml module via compiler option. See this answer for more details.