[polishdude20]

I wonder if it's different if you've studied the language vs if you grew up with it.

I learned polish from my parents when growing up and English from living in Canada and cartoons. My native tongue is English but I can speak polish fairly well and read and write it. I don't ever feel like I accidentally reach for polish or English words when I need the other.

[mejutoco]

If I understood correctly, the parent poster is saying that he mixes words between their second and third languages, not the native one.

I agree with your interpretation though. My anecdotal experience tells me this happens with studied languages.

[RajT88]

Correct - I have trouble staying 100% in non-native languages.