The <Say> verb allows your application to programmatically speak dynamic text over a call or conference using Text To Speech (TTS) capabilities. Voices and languages are declared in your TwiML response using the <Say> verb's voice and language attributes. For help forming TTS responses in your TwiML, please see Twilio Docs: Text To Speech - TwiML
Twilio's Text To Speech offering has a variety of different voices in multiple languages and locales with their associated accents and pronunciations. There are three types of voices with different quality, language coverage and pricing: Basic, Standard and Premium.
For complete list of available voices, please see Twilio Docs: Text To Speech (TTS) - Available voices and languages.