The issue is that gfxboot's gettext support works on "label"
strings but doesn't work properly on "menu label" ones as of 4.04
(the "menu label" translations pop up in the "Loading ..." window
but menu items themselves are unaffected thus untranslated).

NB: debian wheezy's syslinux-4.05 package patchset contains
somewhat related 07-gfxboot-menu-label.patch; might be worth
attention given that debian folks participate in upstream.