Something along the lines in the below diff would make the first
LANGUAGE item (and not the LANG value) be suggested in GDM's language
picker for newly created users:

--- old/daemon/gdm-session-direct.c	2011-03-23 05:50:32 +0100
+++ new/daemon/gdm-session-direct.c	2011-03-23 06:00:33 +0100
@@ -691,10 +691,19 @@
 static const char *
 get_default_language_name (GdmSessionDirect *session)
 {
+    char *langlist, *language;
+
     if (session->priv->saved_language != NULL) {
                 return session->priv->saved_language;
     }
 
+    langlist = getenv("LANGUAGE");
+
+    if (langlist != NULL && strlen(langlist) > 0) {
+        strncpy( language, langlist, strcspn(langlist, ":") );
+        return language;
+    }
+
     return setlocale (LC_MESSAGES, NULL);
 }

Unfortunately the code makes GDM crash at login... I'm probably close,
though, because outside of GDM this test code works without complaints:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

main() {
    char *langlist, *language;
    langlist = getenv("LANGUAGE");
    if (langlist != NULL && strlen(langlist) > 0) {
        strncpy( language, langlist, strcspn(langlist, ":") );
        printf("%s\n", language);
    } else {
        printf("%s\n", "LANGUAGE is empty.");
    }
}