Eh, I don’t know. Most every Parisian speaks some English, but we’ll make it a point of honour not to understand American tourists if there isn’t a buck in it ;).
Yes, this is very common in my experience, too, with my family. English words may get dropped into Polish sentences, sometimes Slavicized to fit the Polish language, sometimes not. Polish words end up in English sentences. And not uncommonly a sentence will start in one language and end in another, or snake back around to the original language. It’s not something I really notice unless I’m paying attention or someone comments on it. My friends were always amused when I spoke Polish with my parents and there’d be sudden interjections of English within the sentences.
Code switching in my household is very common. I speak in Hindi and some Tamil with my mom and dad. In English with my son and English and Hindi with my wife. She talks in Telugu with her mom. A single sentence in any of our conversations can have words from all the above languages.
This must be modern, though. One of my immigrant ancestors was from Bienne, and was monolingual francophone insofar as records and family lore can be believed. They left in the 1830s, though, and I’m guessing that was before compulsory public education.
That doesn’t sound like someone who is bilingual.
This happens to me a lot as well. My Thai may not be all that good, but it’s not that bad. I at least have pronunciations down cold. But I’ve had Thais look at me cluelessly or look at my wife for help, and the wife confirms I was understandable. It’s just some people expect so much to hear English coming out of my mouth that it’s what they really do think they’re hearing. And of course, they’ll tell me in Thai that they don’t speak English.
Well, he can get by speaking Spanish, it’s just slow, awkward, and more heavily-accented than a room full of Pavel Chekovs.
But it seems to be reserved for Americans. Mrs Piper and I have always got by just fine in Paris. 
My grandparents both learned Slovak from their grandparents and parents. My grandfather speaks it and understands it pretty well. My grandmother understands it a lot better than she speaks it, so she usually replies with a mix of English and Slovak.
I’ve found with Parisians that if you try really hard to speak French, and fail, they’ll eventually switch to English out of pity. It’s worked for me every time I’ve been there.
(I don’t actually speak French, I just know how to sound like I’m trying to speak French).
My kids are bi-lingual in English and Hebrew, and switch back and forth seamlessly.
I will never speak Hebrew at that level. There are times when someone will speak to me in Hebrew, I’ll answer in English, and the conversation will continue that way. I understand what is being said to me, but I may not have the full vocabulary to answer back.
Another example of code switching here. Our kids are tri-lingual (English, French and Hindi) and sometimes they will switch languages in the middle of a sentence.
For example, the other day my 5yo daughter saw a man riding a bike without a shirt on and said “I just saw un homme nangoo” (nangoo is Hindi for naked)
In fact not only did she use vocabulary from the three languages, she actually mixed the syntax (the adjective following the noun is a french thing).
You sound like my mother. She’s been here longer than you, so her Hebrew’s probably better, but she doesn’t like to speak it and will switch to English whenever she thinks she can get away with it.
It’s interesting to see the difference between my parents in this regard. My mom worked as a high school English teacher for many years, which means she lived in a mostly English-speaking bubble, whereas my dad is a lawyer (and he served in the army), which forced him to speak Hebrew far more often than her. As a result, his Hebrew is much better than hers, albeit still heavily accented.
I just love the Swiss roestigraben.
I was once hiking across the Diableret massif. I started out having a coffee in one mountain hut that was french speaking, and an hour later went to sample some local alpine cheese in another hut that was german speaking. Turns out that somewhere in between I crossed the cantonal border from Vaud to Berne.
Ah the Roestigraben. Despite what Wiki says, the term was already current in 1970-71 when I lived for a year in Fribourg. But it was not really the Sarine. I lived in the town of Marly (actually, I started out the year living in Marly-le-Petit, but during that year it merged with Marly-le-Grand) and it was thoroughly French speaking despite being on the German side of the river.
Someone mentioned Japan upthread. When I spent a month in Japan, my host told me the story of two Americans touring Japan. One, of Chinese origin, didn’t speak a work of Japanese, but of course looked to be East Asian. The other one was western, and fluent in Japanese. Everywhere they went the one who was fluent made their arrangments/asked questions/etc., but the respondent spoke only the other one.
A Brazilian friend in college used to do this. I’d see her talking to other people - they’d speak in Portuguese, and she’d answer in English.
I asked her about it once - she told me she’s normally reply in Portuguese, but was answering in English so I wasn’t left out of the conversation. She was being polite to me.
Being fluent in several languages is much more common than what the average American would guess. One estimate is that only 40% of the word is fluent in just one language, while 43% are fluent in two of them, 13% in three, 3% or so in four, and 1% or less in five or more:
Actually, I think the family is supposed to be Taiwanese, so if the show is realistic, she would not be speaking Mandarin.
My experience, too. I can sort of fake it in rudimentary French based on knowing English and Spanish and understanding the sound shifts between the languages. When I was in Paris trying to bumble through some French, most waiters would just switch to English. It was like we had a truce-- you can listen to my tortured French, or help me out with your pretty good English. And maybe I was oblivious, but I never felt like they were being snooty about it.
As for the OP, my father was the same way. I never once heard him speak the “Old Language”, but his mother would speak it to him, and he’d reply in English.
Digital is the new Analog’s comment about her roommate always speaking in English so as to not leave her out reminds me of what I’ve been told about the rules (not formally promulgated but just informal rules of etiquette) about speaking English (or any other spoken language) on the Gallaudet University campus. Gallaudet, in Washington, D.C., is the only university in the world where all classes are taught in sign language. All the students and a lot of the professors are deaf or at least significantly hard of hearing. The rule is that all publicly observable conversations are in sign language (perhaps simultaneously with spoken language). The idea is that speaking people are never to look like they are deliberately keeping deaf people from understanding them. The deaf students consider the campus to be their one refuge where they’re not treated as some kind of freaks who can be kept out of conversations.
A member of the book club I belong to teaches at Gallaudet (and she’s not deaf herself, nor is anyone else in our club). Sometimes we have held movie days in a movie theater on the campus on a Saturday. She can get us permission to use the theater for our book club by declaring our movie showings to be campus events where all the students, faculty, and staff of Gallaudet are invited to see the movie too. The movies had to be subtitled in English. Members of our book club introduced the films speaking English while the member who teaches at Gallaudet simultaneously interpreted in sign language. She told us that we were to be careful for us to not all hang out in one group in the theater speaking to each other in English.