This is really simple and the definetely has the function, try the following:
utterance.voice = [AVSpeechSynthesisVoice voiceWithLanguage:@"en-gb"];
If you need to know all the languages avalible, put the following code somewhere:
NSArray* voices = [AVSpeechSynthesisVoice speechVoices];
NSLog(@"Voices: (%d) %@", voices.count, voices);
If you need further documentation on AVSpeechSynthesisVoice
, then look at the following link: