What is Polonius3's name for this linguistic phenomenon?
- Jak się masz?
- Bardzo good.
Forget what P3 calls it,
what linguists call it depends on why it's happened:
if the person who says bardzo good does so because they forgot or don't know the word dobrze and an English word came out (on purpose or accidentally) then that's interference. Nb, if a person is learning a third (or later) language, interference is liable to be stronger from the person's second language than from their first.
If the person knows and remembers the word dobrze but uses good instead for whatever reason, then that's code-switching and tends to be endemic in bilingual settings (where all involved are fluent in the same languges).
NB borrowing an English word and giving it Polish endings (or vice versa) is borrowing. Borrowings range from nonce (happening once) to rare, sporadic, frequent and finally established when you've got a new word in the borrowing language.