Xdg-user-dirs is a tool to help manage "well known" user directories like the desktop folder and the music folder. It also handles localization (i.e. translation) of the filenames.

The way it works is that xdg-user-dirs-update is run very early in the login phase. This program reads a configuration file, and a set of default directories. It then creates localized versions of these directories in the users home directory and sets up a config file in $(XDG_CONFIG_HOME)/user-dirs.dirs (XDG_CONFIG_HOME defaults to ~/.config) that applications can read to find these directories.