I upgraded to macOS Sierra and despite my system being set to French before and after the upgrade, Siri default to U.S. English. I couldn't believe my ears when my first attempt to ask Siri a question resulted in a network error in Franglish:
French Siri does a better job with French error messages.
The upshot is that when there's a network problem, Siri tries to read localized error messages. Once the network problem went away, Siri started answering properly in whatever language I chose, despite the system language being French.
(I am still waiting for bilingual Siri. I have to keep Siri set to English on my iPhone, or she can't dictate messages to my wife in English.)
French Siri does a better job with French error messages.
The upshot is that when there's a network problem, Siri tries to read localized error messages. Once the network problem went away, Siri started answering properly in whatever language I chose, despite the system language being French.
(I am still waiting for bilingual Siri. I have to keep Siri set to English on my iPhone, or she can't dictate messages to my wife in English.)