xfconf-query localization bug

[Keith Hedger]

Found a small but annoying bug in xfconf-query when using a language 
that uses a comma instead of a decimal point, this gives an error:

LANG=es_ES.utf8 xfconf-query -nRt double -c xfce4-desktop -vp 
/backdrop/screen0/monitor0/saturation -s -2,0
Imposible convertir "-2,0" al tipo "gdouble".

This was thrown up in a c program that built a command line via sprintf 
where the float operator ("%f") uses a comma instead of a point.