Well, XBO is at 350$ in US by January, and recently MS have annunced a pricedrop in UK...

But when do you think that will be at 350€ in countries like Italy, France, Germany, Spanish ecc?

What are you talking about? At least in Germany you will have a hard time finding an online shop or retailer where it's not below 350€. At some it's less than 300.

The official price is still 400€.

Official price doesn't mean shit if your product sells so awful that the retailers have to dump it at low prices.

Ms cares only for its strong places,i.e USA/UK but they should definitely do price cuts in other European territories,it's being outsold by the wii u...
They should try,it won't make miracles(Threatening ps4) as the interest for it is not very big in these territories but they will have a satisfying a effect,price cuts always helps a lot...
But I rather be cautious I can't say with certainty if it wasn't cheaper/than ps4 that it wouldn't steal sales from it in many european countries,taking into account the financial problems of the most citizens in several European countries...
In the current situation being cheaper is a merit over the antagonist when you have the most of the things it has...

Ok in this case:

When MS will drop the XBO price at 350€ in Europe?

MS doesn't really have to do anything. The retailers will do the price cuts for them.

no point in lowering the price if it's already lower in reality. So I doubt we'll See an official price cut anytime soon. Maybe around the holidays when demand goes up a little. But I doubt they will stop at 349 and instead go right to 299. a PS4 price cut around the same time is pretty likely after all.