contrary to popular belief, someone who is fluent in their second language (L2) is unlikely to slip into their first language (L1) in these circumstances:
- if someone just said something to them in L2 (this a big unconscious cue, and you’d be really unlikely to respond in L1 right after that)
- when swearing in the middle of a sentence (e.g. “oh merde, i forgot my keys!”)
- during sex
- when speaking to someone they normally speak to in L2
it is slightly more common in these circumstances:
- swearing, as long it’s not part of a sentence (e.g. they might just mutter “merde” if they forgot their keys)
- if they’re surprised (especially if falling/tripping or experiencing sudden pain!)
- when speaking to someone they normally speak to in L1
- in their sleep or talking to themselves
- when very disoriented, such as when concussed or on certain drugs
that being said, it is very common for people to intentionally use their first language in front of people who don’t speak it for a variety of reasons (they might use a short expression they only know in L1, call their partner pet names, dirty talk during sex because their partner finds it attractive) – but this is on purpose!
also this doesn’t account for people who grew up in an environment where people often mix multiple languages in their speech (e.g. spanglish or franglais) – in that case, they may accidentally drop an L1 swear into an L2 sentence, though they’ll still generally stick to L2 when speaking to people who only speak that language
also: if you for some reason forget a word in L2 (or L3 or L4) in many cases your brain will automatically try to recall it from the last language you learned, not necessarily your mother tongue/L1. it’s a really nifty little phenomenon that has the bane of my existence for some time.
also: it’s more likely (at least to me & other bilingual people I know, and ESPECIALLY if your second language is english) for you to slip into L2 while speaking L1, or to pepper L2 terms into a L1 conversation because you happened to forget how to say those words in your native language.
Also if you grew up around multiple languages (especially when they’re being mixed a lot), you might not know for sure which word is in which language. I certainly don’t. Like, I learned in my mid-thirties that something I thought was a Tamil diminutive was actually a bit of Kannada that my family used as a diminutive? And that two words I thought meant slightly different items, were actually just the Tamil & Hindi words for the same thing?
Also! Small bi/multilingual children will mix things up in the cutest way. Hathiphant is my fave example :) (Hathi is Hindi for elephant)
official linguistics post
I once read some complaint about how Hercule Poirot slipped into French at all the most unrealistic times (exclamations in the middle of sentences; epithets for people; greetings; etc.) and my first thought was… He is Hercule Poirot. He was never accidentally slipping. He would not, either accidentally or on purpose, say things in a language his conversation partner would not understand, that would be just counterproductive… which is why he deliberately picked all the French words most likely to be understood from context by a non-French-speaker. And why? Because he’s a vain little silly who’s absurdly proud of his nationality and wanted to show off. So there.