For VL 5.9 in addition to setting the LANG variable you also need to set a few more. These can be added to /etc/profile.d/lang.sh right after the line rbistolfi had you change.
If you don't set all four variables you'll have a system running Spanglish, a little Spanish here, a little English there...
Vector Linux 6 release candidate 1 is out now. It's still a bit rough around the edges so waiting for the final release may make sense. I've run my system in English, French, and Hebrew and it all works easily in 6.0