I'm not able to reproduce the issue. Works fine with <!-- +ěščřžýáíééó --> in arquillian.xml, using RHEL 6.4 with en_US.utf8 locale.
My guess is that arquillian.xml is not encoded with UTF-8. It might also be a configuration issue with Maven - your locale is not UTF-8 enabled and Maven chooses platform locale as default.You can fix the latter by putting <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> into <properties> in pom.xml file.
that was exactly the issue