This is pretty common in many US households where one or both of the parents are non native English speakers. Kids speak English, parent speak whatever non English native language.
My younger kids do it in Chinese with their Mom – and their first language was Chinese but 5 years in China then 5 years in the US has them all melted potted out.
In my office I speak Chinese and everyone speaks back to me in English, I think because of a mutual desire to practice language.
However, if I ever need to switch (e.g. a guy joins the conversation who doesn’t speak chinese), it’s awkward. My brain gets in a particular mode of converting thoughts to words, and I need to load another disk or something to change language.
We have three languages going in our house here in Taiwan.
My Taiwanese wife and I talk in Japanese.
My kids are best at Chinese, second is Japanese and coming up last is their English. My wife will speak Japanese to them so they can continue to retain that language, but they answer back in Chinese.
I talk to the children mostly in English, and they answer back in Japanese because it’s easier than in English.
Occasionally in meetings here in Panama people will speak either English or Spanish depending on their relative fluency. (For me, I may speak Spanish for straightforward stuff but switch to English if I need to say something complicated.)
Some of my relatives speak to me in Catalan when they think they have the upper hand, in Spanish when they want something from me. They’re the kind of people who see everything in antagonistic terms.
With other people, we switch back and forth - sometimes, this may include more than two languages. Conversations with The Divine Victory (one of my college teachers) would require occasional reminders that not all of us spoke German at the level he did: Spanish, English, French and some Latin ok, German please translate.
I’ve been in meetings which in theory had to be conducted in English but people would use their own native language when they couldn’t figure out how to explain whatever in English; sometimes the note-taker needed a translation, sometimes they were able to just take the notes in English of whatever had been said in another language.
There are language-exchange groups which work similarly to what Mijin described for his office, I used to do that with the admins in a German factory. I practiced my German, they corrected the biggest mistakes; they practiced their English, I corrected any big mistakes.
It’s very common among young children who are mixing (even on relatively short holidays abroad) with children who speak another language, even if they’re not specifically being brought up as bilingual; though there it tends to be a macaronic mixture of languages.
When I was at university studying German, there were several lecturers who were bilingual in English and Welsh, including one of my tutors. Sometimes his tutorials would be interrupted by a phone call on some piece of faculty business and the conversation would slip in and out of Welsh depending on how confidential/scandalous the information was.
And upper crust English people used to have a saying “Pas devant les enfants” and sometimes switch into (not necessarily very good) French for much the same reason.
What can be difficult is trying to maintain two conversations in different languages.
Unfortunately, the Cub is going to French immersion, so we don’t have that option in our household. 
I find that it is relatively easy if it’s a noisy group around a table in a good restaurant with the wine flowing. 
I was raised in a bilingual household, in which everyone has a specific language they speak with each specific person:
My mom speaks English with everyone.
My dad speaks English with my mom and me, and Hebrew with my younger brother and sister.
I speak English with my Mom and Dad, and Hebrew with my brother and sister.
My sister speaks English with my mom, and Hebrew with everyone else.
My brother speaks Hebrew with everyone.
My wife has managed to fit in almost seamlessly, speaking English with my mom, alternating English and Hebrew with my dad, and Hebrew with everyone else.
Conversations are completely bilingual.
I tend to automatically adapt to the language the other person is speaking: high German (as opposed to my native Swiss german dialect, English, French, Italian. For example, in a discussion round after a talk I gave to students in France (in English), the students would start asking questions in English, then switch to French as they could not find the right words in English. I would automatically start to answer in French before switching back to English as I could not find the precise expression in French. I have to make a constant conscious effort to not switch languages when talking to a foreigner that wants to learn German but talks to me in an other language.
I used to talk to a coworker in English and she would talk to me in Japanese. No reason not to, if both of you understand each other.
Happens all the time. My parents speak Spanish, I answer in Swedish. They often switch mid-sentence, too, when they need a word or expression from the other language, like:
“… Och det är förstås jättesynd, men ‘más se perdió en la guerra’.”
("… And of course that’s really sad, but ‘more was lost in the war’.")
I once trained a Chinese man at a job and as I spoke to him in English he took all his notes down in Chinese, it was pretty cool.
I have notice Filipinos, in my experience, hold conversations in two languages.
Doesn’t this need to be clarified?
My Boss is a Chassidic Jew. Many’s the time I’ve heard him talking to another Jewish person in English, then switch to Hebrew and back again. Especially when talking about religion.
Whenever my Salvadoran ex girlfriend would speak with her family or other Spanish-speakers, they’d float between Spanish and English effortlessly, sometimes switching from Spanish to English, then back to Spanish in the same sentence. It impressive.
Same here. I speak Spanish considerably better than I can understand it.
i live in southern California and here it’s not uncommon to hear a conversation where one or both participants are speaking in a combination of Spanish and English, sometimes within the same sentence.
I speak English to my mother. She sometimes talks back in Marathi. I don’t speak Marathi (I can count and ask for food and say the other things that kids talk to grandmothers about) but I generally understand everything she says.
Read “with” as “to” and it’s perfectly cromulent.
IOW, Mom speaks English to all, including brother. Meanwhile brother speaks Hebrew to all, including Mom.
When I was a beginning Spanish student, I traveled to Texas. The announcer on the radio would mix Spanish and English in the same sentence. I was lost. When he used an English word, it was as if my brain was searching my Spanish dictionary to no avail. When I finally realized it was an English word, I had missed a few ongoing sentences.
But now, fluent in Spanish, this type of mixing does not bother me. In fact, I think it is kind of fun.