- Many basic expressions might be different, but they're learn-able without any formal education; simply watching TV in the other language constantly for some time allows you to pick up most of these differences
- If you speak slightly slowly, and stick to standard vocabulary, you can communicate with native speakers of the other language without much trouble.
If these above two points are true, then they're about as different as the various Arabic dialects.
Whether you identify these languages as separate languages or regional dialects is largely a political/cultural issue.
There are actually quite a lot of regional dialects within the US, not even getting into the differences with how Canadians, Brits and Australians speak. Some such differences have stalled political negotiations and such historically.
FWIW: I speak conversational German but can't really follow the News in German and can't really read or write it. I read and write French a little but never developed an ear for it so I have enormous difficulty following spoken French and struggle enormously to put together a spoken sentence. I think there are substantial differences between written and spoken, formal and informal for any language.
From my point of view (as an Arab), Italian and Spanish are too similar to be considered separate languages.
It's mostly a matter of culture. We Arabs think of ourselves as one culture. We accept regional differences between dialects as a natural phenomenon. But if you actually think about it, there are big differences in basic expressions and various grammatical structures.
izzaiyak - kiifak
kwaiyis - mneeh
buss - leek (look)
3aiyz - biddi
mush raiye7 - maani raie7 (I'm not going)
Most Arabs know Egyptian and Syrian because they're constantly exposed to these dialects on TV, not because they're intrinsically mutually-intelligible. Izzaiyak doesn't resemble any word in Syrian or Iraqi Arabic, and I can't think of a possible Classical Arabic word as the origin for it, and yet we all know what izzaiyak means because we learned it from TV.
If you move to a region whose dialect isn't popular, you'll find it difficult to understand at first, but you'll quickly be able to pick it up; probably in about a week or so.
In the west, these concepts seem foreign; maybe it has something to do with focusing on individuality over the group.
I know in Japan there are many regional dialects, and they can be mutually-unintelligible at times. The Standard Japanese is the Tokyo dialect. I don't know what it's like if you're a native Japanese, but as a non-native, studying the Standard Japanese doesn't seem to be enough to pick up the various dialects just by ear.
I was watching a Japanese drama once, and some character was speaking a weird language. I thought he was speaking Korean or something, but I asked on a Japanese forum and they said it was a dialect of some region.[1] It seemed even natives have trouble deciphering it.
[1]: http://www.guidetojapanese.org/forum/viewtopic.php?pid=22386...