Yes, that is very normal for multilingual people. It's called code-switching and it has been intensively studied by linguists.

https://en.wikipedia.org/wiki/Code-switching

In linguistics this is called "code switching", and it is extremely common among native bilinguals.

I'm bilingual English and Mandarin and I mostly do this in Chinese restaurants. The real hole in the wall places with the best Chinese food where the servers greet you in Mandarin by default if you look Chinese. Mandarin is more "computationally" expensive for my brain because I'm so used to speaking English so as soon as I have to express something complex I'll just blurt it out in English instead of stuttering it out in Mandarin, which prompts an English response from the server, and we'll go back and fourth switching between the two languages.

I've been noticing that when I read an English text to someone who also speaks my mother tongue, that I will switch to my mother tongue for reading out numbers. For some reason, it feels pretentious to pronounce it in English.

My wife and I are constantly switching between English and French when conversing amongst ourselves. I've often noted that when we want to emphasize a sentence, we use the others native language. It also comes in handy when in public and we want to convey something in secret, because both of our accents in our mother languages are quite strong, so at a whisper even people who know the language but are not fluent will not grasp what we are saying.

Not the same thing, but just the other day I accidentally started speaking French in the middle of a conversation in Spanish, and it took me a minute to understand why the guy suddenly couldn't understand me

Yeah, definitely. Spanglish (Spanish + English) is very common in California.

Yes, my mother tongue is English, and I spoke some Spanish before I began immersing in Italian. At one point, I couldn’t stop speaking ItalSpanGlish. Nobody could understand me.

I speak english, italian and arabic. and it's so much fun to switch between the three with other multi-linguals, personally sometimes i find it hard to switch to english after speaking italian for a long period of time, and when i read english text i tend to pronounce the numbers in italian as it feels much easier and makes more sense for me.

I think it's overall a fun experience.

Frequently mix my native and second language and sometimes can’t think of a good enough word in my native language. Rarely mix my third language except when speaking to people of that country or in that country.

English, Spanish/Spanglish, and a bit of French. Parfois, I like to cambiar idiomas dans le middle of une phrase to mess with mes amigos. But yes I always mix up things. The most common is when I accidentally inject Spanish into French

I remember a subreddit called "Belgicaans". It's basically a place where we would converse in sentences that combined our three native languages (Dutch, French and German). Was very funny to do actually! But speaking like that? No, never

Same, (in that I speak both Portuguese and English) and that I do sometimes feel the need to jump around

TurkoGerman here and we do that all the time. Our families back in turkey learned enough german by now thay we even do it in turkey...

Same for my tunisian wife (Arabic instead of Turkish though).

Being English, I'm stubbornly monolingual (aside from some leftover schoolboy French), so when I was invited to a Sikh wedding I was genuinely amazed by all the guests just flowing between English and Punjabi as if they were the same language.

Yes, every mac madra does this

Have a relative who has another language as their mother tongue but has perfect of my native language so we code switch a lot, I'm not fluent but I enjoy throwing in bits of what I know and they seem to appreciate it and encourage it

Weirdly, I kind of like parts of their language so I have lots of fun with wordplay and playing with the spelling and such and mashing them together for comedic effect like Sacha Baron Cohen does in Bruno

Yes, eng fr spa, mix all sorts of words together or use gendered words ungendered when using them in English

Quite common in India to speak a mix of English and another language. My partner and I mostly speak English, but some sentences just happen in Hindi.

Hardly ever, but sometimes I find grammatical, syntactical, etc. elements of the other languages subtly bleeding through in my writing and speech. e.g. habitually writing "1.", "2.", and "3." instead of "1st", "2nd", "3rd", even for an English piece.

Maybe it stems from the way I acquired my languages. Code-switching tends to throw off my thought process, especially if I am the one doing it. I'll have to finish a thought (or an entire chain of thoughts) in one language, and only then will I have an opportunity to switch the language.

I’ve only ever heard this in American movies by hispanic characters. To me, it would feel extremely pretentious to do this in real life

White washed (westernised) Arabs do usually speak in a specific language but substitute specific words in the other language
I really hate this though as it causes the speaker to only know each word in one of the language which basically means they can speak neither properly