Thanks to all, namely ROARDE!
I appreciate the good word, but no: the thanks go mainly to you, kolaloka
You see, this is something that's needed to be looked into for quite a while. I'd intended to compare STD and Light on my way to suggesting a fix. I'll skip the details and just say that would have involved a lot of time in my situation, probably 3 days or so and no telling when I'd get to it. It wouldn't have taken someone else quite that long, but still a fair amount of time. Instead, you looked at it well, reasoned it out right given just a hint, and published the results in very clear fashion. So again, thank you very much!
I'm not a VL dev, but I can guess that there won't be a package-type or automated fix for 7.0. It probably won't make a 7.1 beta very soon either, but I'd bet it will be an improvement included with 7.1 release.
The only thing that remains unclear to me is why VL 7 LIGHT ignores LANG=cs_CZ in /etc/profile and introduces GDM_LANG instead?
Most if not all the desktop environments shipped with Light 7.0 miss running or including /etc/profile or .profile. To me, it's not certain that profile is the right place to get this for X anyway, but I haven't seen any distro with a proper alternative yet. Lxde was pretty new to VL when it was shipped with Light 7.0, but all in all a pretty good job. But if you'll look in lxde's startup directory (I run IceWM and can't recall where that directory is), you'll see a shell script intended to be a startup file for lxde. Lxde wants .desktop files there instead, so the script isn't run. It can be run just by creating something like "startscript.desktop" in that directory; include the line "Exec=/path/to/shellscript". Some of the entries on the existing script are duplicated by actual .desktop files or elsewhere, so an edit would be needed before activating the script.
I hope my post may contribute to improving VL. :-)
It has, in a very big way. 4 posts, 2 fixes (counting your other thread), and much effort saved. Nice average!