IBM Bluemix Watson Text-to-speech service supports Mandarin and Cantonese.
IBM Bluemix Watson Speech-to-Text support Mandarin, it's logical that Text-to-speech support Mandarin.
Meanwhile, there is a niche market on STT/TTS for Cantonese, which could serve Hong Kong, Macau & some south east Asia customers.
NOTICE TO EU RESIDENTS: per EU Data Protection Policy, if you wish to remove your personal information from the IBM ideas portal, please login to the ideas portal using your previously registered information then change your email to "email@example.com" and first name to "anonymous" and last name to "anonymous". This will ensure that IBM will not send any emails to you about all idea submissions