I've been asked if I ever confuse languages, with all the study I'm doing in different languages. The short answer is, for the most part, no, not at all (with a few caveats). I can keep different languages all compartmentalized in the right places, for the most part. And I think most people who have knowledge of different languages don't have that trouble in general. Let me ask you this...do you try to play guitar chords on the piano but then get confused because there are no strings readily available (although I have tried to find techniques of piano playing that involve plucking the harp, but that's a totally different thing)? Do you ever accidentally cook using a sponge instead of a spatula? Can you tell your parents apart?
OK, those are extreme examples. But I think you get the point. You get used to using a tool in a certain context, and you associate it with that context. Vocabulary, grammar and speech are tools of communication, and they happen in boxes that are confined to a certain linguistic context.
But there are caveats to that general rule. Sometimes I find that I can't think of the word in the right language, and the only word that will pop into the forefront of my mind is the word in a different language. And I want to use that word, or sometimes even will in frustration, but I'm pretty aware that my target audience may not understand it.
Or sometimes there are similar words in languages from the same linguistic families. Like "ancora" in Italian and "encore" in French (again, yet, still). Or "donde" in Spanish and "dove" in Italian (where). "Hablar" in Spanish and "falar" in Portuguese; "parler" in French and "parlar" in Catalan (to speak). I could possibly wrongly throw in a Spanish cognate when I'm speaking (or, really, butchering) Italian. But I'm not going to confuse French with Mandarin.
When I was first learning Polish, for some reason, I kept wanting to say, "wo ist" instead of "gdzie jest" (where is...). But I knew that tossing German into the mix was not the right thing to do. My brain just kept wanting to pick that as the first choice, even though I knew it was not right.