Jim, thanks for pointing me to the right direction.
Indeed locale is set differently for cron compared to my shell environment.
I altered my script so that it runs
LC_TIME=en_US.UTF-8 who -u ...
If I run 'who -u' interactively or from a script invoked through bash in a tty on my Ubuntu 12LTS box I get an output like this:
testuser pts/0 Dec 9 02:32 . 2163 (host.xx.yy)...