I dated a girl in college who spoke fluent German (she was American), then learned French. When she got drunk enough, she’d begin a sentence in English, switch to German, and finally, end it in French.
I miss her.
Grosie, my paternal grandmother was fancy Pennsylvania Dutch and many of her conversations to her kids and grandkids began Mein got im himmel. 
RivkahChaya, the same (gentile) grandmother also used farkockteh, so it likely had origins in old low German. There were words that she never did use English for.
Both of my Latino BILs have done the code switching with Spanish, with Sis and I occasionally catching ourselves doing it too.
Switching languages back and forth is very common among my Uber passengers. I guess that there are just some ideas more easily stated in one or the other language.
I worked for a while in a factory in the Black Forest and used to have lunch with several of the administrative staff, trading English-improvement for German-improvement. When we code-switched a complement and there was a split verb, it would be German, then the complement in English, then the second half of the verb. But as in your friend’s work, a whole “chunk” of German got substituted by its English equivalent.
The adjective/noun thing is more complicated, in that while an adjective is a syntagma by itself, the adjective+noun(+article if any) form a higher-level syntagma; also, while in French the adjective usually follows the noun, it does not have to be so and vice versa for the order in English. Mixing noun and adjective in Fr+En, Es+En, It+En can be done, but it makes the higher-level syntagma “screech” and it tends to be early stages in the adoption of a neologism, rather than pure code-switching (if there is such a thing, it kind of sounds like an oxymoron but I don’t think it is).
I am not the OP, but growing up in a monolingual English-speaking environment, I found code switching strange when I first encountered it in Africa. I thought it would have been easier to stick to one.
It’s so common in Africa that you could almost say that it is rare to hear someone not doing it.
A friend of mine does this a lot when other Spanish speakers are around. He’ll sometimes do it with me forgetting I don’t comprehend spoken Spanish.
ETA: And I have a Mancunian mate who does it with his fellow gibberish speakers forgetting that the rest of us only understand forms of English used in almost all the rest of the world.
I work in a store owned by Chasidic Jews and with Spanish speaking employees. The back and forth between Hebrew, Yiddish and Spanish astounds me.
Something the monolings are very mistaken about is that they think that speaking another language is hard. It’s not. Multilingualism is the default human condition. They’ve just been deprived of this experience.
My wife is Taiwanese, we met in Japan and our common language is Japanese.
Both our children were born in Japan and my wife continues to speak Japanese to them so they don’t forget.
The children speak Mandarin to each other as well as to their mother who answers back in Japanese.
I speak to them in English and they talk to me in Japanese.
I actually trained myself to not switch into Japanese with them.
Think of it as being a wider vocabulary. Specially when the grammar systems involved are relatively close, that’s pretty much what it ends up amounting to. Someone who only speaks a pidgin is less flexible than someone who speaks a non-pidgin language correctly; they’re as limited as anybody else who can’t switch register at all. But someone who speaks two languages correctly has twice the vocabulary of someone who speaks either one at the same level as he speaks those two.
There’s a reason that terms like “Spanglish” and “Chinglish” exist, and probably many many other examples. I always find it entertaining listening to some of my calculus students bounce back and forth between Korean and English, usually using English for the jargon of the subject.
Nitpick: “Chinglish” is usually used in a pejorative sense, referring to English text that has spelling, grammar and/or word choice errors. It’s rarely used to mean a fluid mixing of fluent English and Chinese.
(If you’re wondering if there’s a word for bad Chinese spoken by English-speakers e.g. “Englese”…I don’t think so.
While English speakers obviously often make mistakes when trying to speak Chinese, there simply aren’t enough foreigners, comparatively, learning chinese for it to be a named phenomenon yet.
Also, the economics are different: a mom and pop store in the US is unlikely to choose a Chinese name to try to seem upmarket, or translate signs into Mandarin to cater to Chinese customers)
And so’s Spanglish; it doesn’t refer to people code switching, but to what’s spoken by people whose “English” is not English and whose “Spanish” is not Spanish.
A coincidence today (before I read your post) relevant your story in particular.
As you say, it’s polite to not speak solely in a foreign language when someone unfamiliar with it is in earshot and can be expected to be re-included in the discussion.
The etiquette is (for good and interesting reasons) significantly more strong, mandatory, for anyone “well brought up” when speaking in front of a someone who is deaf–even in the same room with, let a alone a three-person interaction. It becomes automatic to sign and speak simultaneously.
Today I was talking to two friends, one of whom speaks Spanish and English, the other only English. In walks a new person–oh, you’re from Spain! I love to hear the Castillian accent, lets blabber!
After a few sentences I realized, in order, 1) friend’s wife did not seem to be following my excitement in the topic, 2) duh, she doesn’t speak Spanish, 3) I was being boorish.
Very weird few seconds (less probably) passed: my next sentence I said in Spanish and mentally verbalized in English, which I don’t like to do and don’t need to normally, and then was totally flummoxed: I was completely flummoxed that one person, me, cannot do simultaneous translation, when I do it all the time with people who don’t speak English (signers). Works for one code switch, right? so my mind just went with body memory but came up empty.
It was a very weird, almost a moment of paralysis and mental confusion, but fascinating. I’m still trying to work it out, relative to my life-long mental/physical conundrum requiring training and reaction, hearing music in my mind and playing piano.
I’m surprised that these are considered pejorative. In India, people freely use terms like “Hinglish,” “Bonglish,” “Tanglish,” etc., to describe the hybrid languages they habitually use. They are all educated in English-medium schools, so there’s no implication that their English is substandard.