By Supantha Mukherjee STOCKHOLM, Aug 18 (Reuters) - Klarna, the Swedish "buy now, pay later" services provider and online bank, cut its ‌full-year volume and revenue forecast on Tuesday, citing conditions ‌in Germany, its largest market, sending its shares down 17% in premarket trading. The guidance ​overshadowed its surprise second-quarter profit, while analysts had expected a net loss, helped by growth in its U.S. markets. The company now expects its full-year gross merchandise volume (GMV), a met ...