I'm standing in line at my local bakery in Paris, apologising to an incredibly confused shopkeeper. He's just asked how many pastries I would like, and completely inadvertently, I responded in Mandarin instead of French. I'm equally baffled: I'm a dominant English speaker, and haven't used Mandarin properly in years. And yet, here in this most Parisian of settings, it somehow decided to reassert itself.

Multilinguals commonly juggle the languages they know with ease. But sometimes, accidental slip-ups can occur. And the science behind why this happens is revealing surprising insights into how our brains work.

Research into how multilingual people juggle more than one language in their minds is complex and sometimes counterintuitive. It turns out that 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. And interference can manifest itself not just in vocabulary slip-ups, but even on the level of grammar or accent.

https://www.bbc.com/future/article/20220719-how-speaking-other-languages-changes-your-brain