I'm Dutch, my girlfriend is Croatian, we speak English to each other, we live in Costa Rica where my daughter goes to school as well.

I speak Dutch to my daughter. My girlfriend speaks Croatian to her. My daughter goes to school in Spanish. Setup with grandparents is the same as with me and my girlfriend.

We're mostly consistent (though occasionally my daughter asks me to read something to her in Spanish or Croatian, and similar for my girlfriend). It generally just works, my daughter speaks Dutch, Croatian and Spanish well - she plays in Spanish, and sometimes answers in Spanish as well even if we speak our own language to her, but mostly she switches language based on context. Surprisingly (to us) she has absorbed a lot of English as well and generally understands what we're talking about, though we've made no particular effort to teach her that (she gets basic English in school, but it's not much, she's 3). Occasionally she uses a word from one language while speaking another, which can be funny because she does adjust the conjugation, so she'll apply Croatian cases to Dutch words and things like that.