"when a multilingual person wants to speak, the languages they know can be active at the same time, even if only one gets used. These languages can interfere with each other, for example intruding into speech just when you don't expect them."
In Australia I often cause confusion when I switch to Hokkien/Mandarin or Malay midway. Sometimes I don't even realise I am doing this. In Malaysia most of us speak like this, inserting random words from languages all the time.
https://www.bbc.com/future/article/20220719-how-speaking-other-languages-changes-your-brain