I was disappointed last night testing multilingual dictation.
It works, in that you can choose up to 4 languages in Settings, and, for any text input, you can dictate in one of the 4 languages, and it will, after a few seconds, figure out which one you're using. Last night, I tested English, French, Spanish, and Italian.
But it cannot (1) deal with mixing languages. (So no Spanglish!) In order to switch languages, I had to end the input session and start another one.
And (2) it seems that Siri can still only send messages in the language Siri is set to. I tried having American English Siri send Spanish messages, and the results are as funny as you'd expect. And If I set Siri to French/Spanish, she can't handle messages in English or Spanish/French.
Multilingual Siri message dictation is what I am waiting for. They have the capability now; they just need to integrate it into Siri.