There's now a bug report for the translations that don't show up in gpicview:

Two strings also were not marked as translatable in lxpanel's launcher plugin.This may have been done accidentally but I reverted it, built lxpanel here and the translation and the plugin seem to be working fine as usual. But this depends on the file that was removed from the git code some commits ago (I'm not a programmer but from my tests, removing the glade files makes those parts of lxpanel not translatable):

lxsession-edit's desktop file isn't being translated too. I told Julien that and some time ago he said he had a fix for that and would commit it but I'll mention it here because it hasn't been done yet and, IMO, all these translation related bugs should be fixed before a new release of these apps is launched (and they're apparently all easy fixes as even I could figure).