That laugh and the accents

Because it's meant to portray the Germans as more down-to-earth and realistic (or more crass) relative to their intellectual, foppish Italian counterparts. The simplest means for differentiating between the two nationalities is to give different nationalities different accents. The obvious choices of accent for an English-language film are generic American and generic English. Within the English-speaking world, each accent has a different connotation. The filmmakers exploited those connotations as rough approximations of the cultural connotations of each character's nationality: the Italians come across as stuffy and elitist; the Germans, as worldly. It's subtle, and apparently a lot of people didn't understand the point of it, but the subconscious effect is undeniably successful. It adds another layer to every bit of character interaction and affects their relationships to each other. If everyone had the same accent, the effect would be lost. Similarly, for an English-speaking audience, using correct foreign accents (Italian and German) would completely fail to convey the proper effect, and would come across as quite silly. But using English and American accents adds that extra dimension. It's a bold choice, and one I think worked quite well. Even if people don't understand the effect, they feel it.