I recently came across the “complementarity principle” in bilingualism, the idea that “bilinguals usually acquire and use their languages for different purposes, in different domains of life, with different people. Different aspects of life normally require different languages.” 

This is why bilingual children can end up switching mid sentence from one language to another. If you’ve ever heard a bilingual second generation immigrant talking to their parents in their parents’ native tongue, you’re likely to hear English words in the mix too! I certainly intersperse Italian with bits of English when talking to my parents. 

On a related point, studies also show that adults who learn new languages feel less emotionally attached to them. 

For a bilingual adult listening to swearing in both languages, swear words in their native tongue provoked a much stronger physical response. 

It seems that even if you understand the meaning of new words, your brain still feels them differently. This explains why Jurgen Klopp, Liverpool Football Club manager has a tendency to swear in English on TV! He may feel less emotionally attached to swear words so is prone to lapses. I suspect he doesn’t swear as much if at all when being interviewed in German.