>> To my surprise, she doesn’t mix words from the different languages in the same sentence

I knew two brothers that would mix words from different languages while speaking to each other because they shared the same set of languages and presumably used the best words to express their thoughts.

Your daughter probably knows other people generally speak and understand one language at a time and just conforms because its most effective.

I'm not sure if or at what age it might be good to start mixing languages with others who can.