dbus_backend/ls-dbus-backend in the D-Bus backend in language-selector before 0.6.7 does not restrict access on the basis of a PolicyKit check result, which allows local users to modify the /etc/default/locale and /etc/environment files via a (1) SetSystemDefaultLangEnv or (2) SetSystemDefaultLanguageEnv call.
https://launchpad.net/ubuntu/+source/language-selector/0.6.7
https://launchpad.net/bugs/764397
http://www.vupen.com/english/advisories/2011/1032
http://www.ubuntuupdates.org/packages/show/307975
http://www.ubuntu.com/usn/USN-1115-1/