accountsservice: shell injection via attacker-controlled ~/.pam_environment in Ubuntu language helper scripts

The Ubuntu-specific language helper scripts (save-to-pam-env, update-langlist) shipped with accountsservice before 23.13.9-8ubuntu7 treat the user-controlled LANGUAGE entry in ~/.pam_environment as trusted input. The value is interpolated unescaped into a GNU sed replacement expression, allowing an attacker to inject a sed 'e' flag and arbitrary shell commands that execute with the privileges of the AccountsService helper process (real UID 0) via the SetLanguage D-Bus method.