What is the current price per litre of diesel in France and what is the best web site to check daily?
It might be kinda tough to find a website that'll update fuel prices in France daily, especially in English. I usually use the Automobile Association's website to get their monthly surveys. I was in Spain 2 months ago, and April's average is only one Euro cent per litre higher than it was then. The value of the dollar against the Euro will swing more rapidly that the cost of fuel. http://www.theaa.com/motoring_advice/fuel (go do Download the Full Report, in pdf format.)
There is one: http://gasoline-germany.com/international.phtml?changeto=ENBut I would consider this to be a rough guidline as prices vary so much between urban and rural areas etc etc.
This site - although in French - is the official statistics. Diesel is "gazole". Tick gazole and then a "departement" on the map, and it gives you prices for a large number of gas stations.