If you’ve got files encoded in a particular way, it’s probably best to tell javac that rather than forcing the whole JVM to use a particular encoding. The javac task has an encoding attribute for this reason.
<javac srcdir="${src.dir}" destdir="${build.classes.dir}" encoding="iso-8859-1" />
But really, you should just convert the source files to UTF-8. Everything tastes better in UTF-8. 🙂