I had the same problem when I upgraded from 3.0.4v4 to 3.1.1 on a fresh WP install so
no plugins were active and using the default theme. Tried re-installing the wp-admin and wp-includes but no effect. The solution was that given by godlikemouse. I didn’t create a symbolic link but just copied the wp-langes-en.js as en.js.
Many thanks godlikemouse.
