Earlier quoted context omitted.
A Yiddish scholar once said that "a language is a dialect with an army and a navy." It's really true. One might think that mutual intelligibility (ability for speakers of one to make themselves fully understood by speakers of the other) would be the standard for differentiating languages from dialects, but the distinction really ends up just being political. The spoken forms of Hindi and Urdu are entirely mutually in…
A side note: as a teenager I spent one summer working in a clothing store in London, with a bunch of Spaniards and Italians. They spoke to each other using their respective tongue, and said they could understand each other. Just basics of course. Maybe someone from Spain or Italy could chime in here?
Regarding the Spanish/Italian example, the father of a friend of mine was fluent in Spanish, French, and Portuguese. Using an Italian grammar guide and his knowledge of Romance vocabulary from the other three languages, he learned Italian in a single flight from the US to Italy to the point where he was able to conduct business in Italy immediately upon landing.