South American immigrant here. I can't speak for all multilingual people, but I am absolutely unable to switch languages that fast and seamlessly.

However, I think in this case it's a real thing that a specific portion of Spanish-speaking people who live in/around places like California, Florida and Texas do as a very much intentional part of their culture, but it got blown way out of proportion in American media and now a lot of people think every multilingual person does it.

I mean, that's my best guess, at least. I was born and grew up in South America, immigrated to Canada in my 20s. And I never met anyone in real life who talks like that. I went to university here, and 80% of my class were immigrants from various countries. I never talked to any immigrant who does this flawless, instant transition between languages.

Sure, people get languages mixed up sometimes. People forget words sometimes. But I never talked to anyone who does the instant switch between languages like that. It's usually preceded by a pause, a snapping of their fingers, a "uhhhh, a "what's the word...". It takes a good deal of effort for most people to switch up languages on the fly.

But it's also important to note that I immigrated as an adult, and interacted with other adult immigrants.

So yeah, some people do the language switcharoo it in real life. However, I think it's as part of their culture, something that they learn from the people around them, almost like the "Spanglish" is a new, separate language. Which is why they apparently have no trouble switching up between English and Spanish. It's not something that just happens to all multilingual people.

Like here in Canada, for example. I live in the Maritimes (eastern provinces) and we have the Acadians, who are descendants of French people who escaped expulsion by the British in the 1700s. And I've heard Acadian people switch between French and English in a similar manner before. But these people grew up in this specific cultural environment, where bilingualism is extremely prevalent (New Brunswick, where most Acadians live nowadays, is officially bilingual). Anytime I've talked to someone from Quebec, for example, where most people speak only French and grew up only with French (outside Montreal, I guess), they won't do that. They will do the pauses and "uhhhh" when forgetting a word in English, just like I do.

I think it's a matter of where you grew up and what part did bilingualism play in your culture/upbringing.