I know a little bit of Russian so a random word or two sometimes sneak into my speech or thinking process. I imagine if I were fluent in Russian, I'd switch back and forth just as well. They're more similar to each other than Polish and English, but not
that similar as to be bucketed as one language.
I've been thinking about this phenomenon a lot in the past and I've noticed that two main triggers are a) emotional payload of a phrase, and b) subtle differences in meaning of words. Some phrases sound better to me in English than in Polish, and it's probably driven by the context in which I've learned them. As for words, it's often the case that even if a word in Polish corresponds directly to a word in English by dictionary, their etymologies and general use patterns draw slightly different borders in thingspace. Sometimes that distinction is important, and I automatically pick the language in which the word is closer to what I mean.