Ah! I configured it again with the en_US.UTF-8 and had no success. I rebooted in Maemo to connect the µSD to my computer in storage mode, and did a "sudo chmod -R 777 ." in /media/KaliN900 (where the corresponding partition is mounted on my computer). I rebooted the phone, launched Kali N900 again, was not asked to configure anything, and now I have an UI and can enter my username and password! I doubt what I did is safe though, but it might help pinpointing the issue. Any advice would be welcome. Turns out I might have broken sudo by doing that, among other things. (And I have the same Netdev issue as you.)
4755 root:root /usr/bin/sudo 0440 root:root /etc/sudoers