Comment # 4 on bug 898877 from Thorsten Behrens

This is really a desktop feature, as such I hesitate to touch any low-level
utility here. Furtheron, from the xdg-user-dirs-update man page:

 "xdg-user-dirs-update is normally run automatically at the start of a user
  session to update the XDG user dirs according to the users locale."

Which makes sense, also for changing the locale later on.

Question is: why is that not working for your case?