Neither the user of this user account, nor the admin user, has ever explicitly set a locale to en_US. So apparently there was a bug in a previous software version that produced the "en_US". (The same /home partition is being used as when a previous distro was running on the machine.)
In my case, the problem was a file ~/.dmrc which was as follows:
[Desktop]
Language=en_US
Langlist=en_US:en
LCMess=en_US.UTF-8
Layout=us
Session=xfce
Deleting that .dmrc file fixed the issue.
Neither the user of this user account, nor the admin user, has ever explicitly set a locale to en_US. So apparently there was a bug in a previous software version that produced the "en_US". (The same /home partition is being used as when a previous distro was running on the machine.)