Code-switching - it's not uncommon, but also not universal amongst multilingual speakers.

I think bilingual in the linguistic sense is more if you grow up using both languages a lot in early childhood, and especially if you use both in the home, and not so much about the final fluency. I do this kind of rapid switching between Dutch and English because I grew up speaking Dutch at home in an English speaking country. But I wouldn't say my Dutch is really perfect, I am missing quite a lot of vocabulary after decades in an English country and I don't know a lot of formal/polite speech. But I can easily relate to how they are mixing the languages, it is quite natural. I also find it disorienting to switch to languages I learnt later in life, even one I learnt to a very high level, and I don't often mix at all with that one.

Latvia is funny one, like speaking in Latvian with some English sparkles, greeting in Italian and cursing in Russian

Oh, and there's the infamous "ok labi davai čau" that manages to cram 4 languages into a goodbye when closing a phone call

The western Mediterranean is a huge mix anyways. Spanish has many words of Arabic origin. Darija has many word of Spanish origin. Spanish ojalá for instance is just inshallah. Italians and Spanish usually can speak with each other in their respective language. I had a conversation once with a Moroccan living in Italy using my poor level of Spanish. Southern French youth is increasingly using Arabic words learned from the Diaspora.

I wouldn't be surprised if the mix ends up with a new common language being formed in a few centuries.

It's called code switching, and apparently it's common in bilingual communities. On the topic of Arab code switching, rich Egyptian do it too but with English. Now that I think about it rich people not code switching is probably only an Arabian Peninsula thing.

It's called code switching and is extremely normal in bilingual communities.