Specify LC_NUMERIC environment variable in desktop file

In version 0.9.4.183 I have to set LC_NUMERIC=en_US.utf8 to get the Discover page working reliably.
This was suggested by Spotify employees in this thread: http://community.spotify.com/t5/Help-Desktop-Linux-Mac-and/Discover-page-and-radio-page-not-working-on-Linux-on-my-desktop/m-p/503548#M57375