user.language and user.region in systemPropertyVariables don't set default locale

Surefire provides a way to pass system properties to the JVM that runs tests within the systemPropertyVariables section. But it appears that these properties are not actually passed correctly and, for example, the default system locale is not set when user.language and user.region properties are specified this way. A workaround exists, which is to use:

<argLine>-Duser.language=en -Duser.region=GB</argLine>

instead, which should be exactly the same. In fact, when both are used, a warning is issued that one of these is redundant.

This is a "known" problem in the sense that it has its own Stack Overflow page: https://stackoverflow.com/questions/8901880/set-surefire-plugin-locale-in-maven-pom-file.