* Champagne sales volumes seen down 3 pct in 2012 * Revenue seen flat or slightly up from 2011 level * Europe sales down; demand grows in U.S., Japan, China PARIS, Dec 21 (Reuters) – Depressed by weak economies andthe lingering shadow of a sovereign debt crisis, Europeans arestruggling to find good reasons to crack open the bubbly. The European gloom means that after two years of increases …