Although the Shard is very tall and it would seem like you would be able to see France from the Shard it's actually not possible at all to see France from the Shard.
France is more than 100 miles away from the Shard which makes it impossible to see France from atop the Shard.
Now if the Shard was closer to France then you could see France from the Shard but it would have to be around 40 to 50 miles or less to each other to be able to see France from the Shard.
The view from atop the Shard is great though and well worth going on.