Siri’s accent is automatically set to American English for those who buy the iPhone in the United States, but being a multifaceted personal assistant, she can put on a foreign accent at the drop of a hat. Under Settings look for Siri.
From there, a Language option gives you five choices as to Siri’s vernacular: English (Australia), English (Britain), English (United States), French, and German. The two latter options change Siri’s entire spoken language.