[PaulDavisThe1st]

a friend of mine who grew up in argentina went for an interview at a university in brasil.

they reported that on the first day, portugese was just gibberish. on the second day they realized they could read a solid chunk of a portugese newspaper. on the third day they felt they were beginning to understand what people were saying to them.

obviously, YMMV (and does).

[rvense]

This really what "mutually intelligible" means, I'd say: that the languages are so close you can sort of work it out without explicit instruction. You still need some experience with the other language - and quite often there'll be a geographical and cultural proximity that means almost all speakers have that.

I grew up in a Scandinavian country and visited the others a lot when I was young, and I find I understand most of what I hear in the other languages, but it's quite common for my peers who don't have that experience to understand nothing.