This paper describes speaker discrimination experiments in which native English listeners were presented with natural speech stimuli in English and Mandarin, synthetic speech stimuli in English and Mandarin, or natural Mandarin speech and synthetic English speech stimuli. In each experiment, listeners were asked to judge whether the sentences in a pair were spoken by the same person or not. We found that the results of Mandarin/English speaker discrimination were very similar to those found in previous work on German/English and Finnish/English speaker discrimination. We conclude from this and previous work that listeners are able to discriminate between speakers across languages or across speech types, but the combination of these two factors leads to a speaker discrimination task that is too difficult for listeners to perform successfully, given the fact that the quality of across-language speaker adapted speech synthesis at present still needs to be improved.