Siri isn't bi-lingual or even tri-lingual, the best word to describe Siri would be to call her (or him) a polyglot, or one who has mastered many languages, as she heads towards mastering Shanghainese. 

Apple's voice assist currently knows 21 languages localized with different dialects for more than 36 countries and regions, according to Reuters. Siri is exception, even among peers. Amazon's Alexa can only speak English, Google's Assistant speaks English, and German on pixel phones, while Microsoft's Cortana has mastered eight languages in 13 different regions.  So how does Siri do it? 

It's no easy process. It starts with people. Apple first brings in real people to read and record set passages to give their systems accurate transcripts with corresponding recordings, the head of speech at Apple, Alex Acero told Reuters. The next step is to enable dictation, allowing Siri users to dictate to their phone the text they want typed out. Apple saves some of those, keeps the dictations anonymous, but has people translate them to help their system learn even more. 

Once the most common questions and responses are recorded and incorporated, Apple releases Siri and it continues to learn once it's in use and only get smarter.

It's not a fully automated process yet and involved far more input from humans than it will in the future.