Where does that work, and where is that documented?
Where it works: here on Ubuntu, when I say export LANG=de and start inkscape, I get (inkscape:8421): Gtk-WARNING **: Locale not supported by C library. Using the fallback 'C' locale. with no result. But using export LANGUAGE=de (after reboot or on another terminal) all is working expected.
Also ask ACSpike or Ted Gould: they could solve their problems only with LANGUAGE.
I don't care about documentation of it but google gave me http://www.gnu.org/software/gettext/manual/html_node/gettext_155.html wherefrom it seems that it has something to do with priority?
'LANG' is all throughout standard man pages on many OS's, but I've not yet seen 'LANGUAGE'. Explicit documentation on it would probably help understand the subtleties. (last I saw, LANGUAGE was an application-specific thing that they could treat however they liked)
I hope this setles the matter. Please also update inkscape's documentation in this regard.
ralf