That's because the label is hardcoded in the config file (button_title entry in ~/.config/xfce4/panel/xfce4-menu-*.rc or /etc/xdg/xfce4/panel/xfce4-menu-1.rc).
That's because the label is hardcoded in the config file (button_title entry in ~/.config/ xfce4/panel/ xfce4-menu- *.rc or /etc/xdg/ xfce4/panel/ xfce4-menu- 1.rc).