Following up, here is the situation:
Using "Chinese Traditional - Changjie" as input method,
1, dictation WILL listen as Cantonese if SIRI is set to Cantonese-Hong Kong
2, dictation defaults to Mandarin if SIRI is set to English
Further testing concluded that regardless of the input method, as long as Siri is set to Cantonese-Hong Kong the dictation can listen to cantonese, and otherwise it cannot.
Is there a way to individually toggle dictation language?