po: renamed 'locale' dir to 'po' and 'fr_FR.po' file to 'fr.po'

It seems that a 'fr.po' file has already been imported on Launchpad
and it's maybe better to first create a generic 'fr' translation.
About the 'po' dir, it should not change anything but most project seems
using this name.

For more details, see my previous email ;-)