Q: What's the difference between a language and a dialect?

A: A language has an army!

Examples: The Chinese government usually just talks about "Chinese". Mandarin is an English word to describe the Chinese language spoken in northern China--- but the Chinese government as political reasons to try to pretend that Cantonese is just a dialect.

Swedish, Danish and Norwegian are mutually intelligible, but each country has a government department dedicated to vetting books published in the language, and claim that what they speak are languages, not dialects.

I think the real answer is that taxonomy is difficult.

Mutual intelligibility between Danish and the other two Scandinavian languages has declined over the 20th century. Danish has undergone some striking phonological developments (compare modern pronunciation to that of reference works written before the war) that have made it extremely difficult for Norwegians and, even more, Swedes to understand Danish without prior experience with the language. Nina Grønnum's 2003 paper titled “Why are the Danes so hard to understand?" looks at some of these quirks in detail.

It is interesting to attend international events and watch the Scandinavians in their 60s or 70s speak among each other in their own languages with seemingly no problem, since their generation was pushed to overcome any hurdles to understanding the neighbours, while a group of younger Scandinavians that includes Danes is likely to switch to English because it just feels easier.

The 'Chinese' situation is more complicated than that, and partly because the term typically translated as 'dialect' is more aptly translated as 'topolect' (the language of an area). While the Chinese government's language policies are shitty, fāngyán/topolect is, if anything a much more neutral term than 'dialect', which as a whole lot of extra baggage.

Where is the line between dialect and language?

I'd be more inclined to call Italian and Spanish the same language than Spanish and Portuguese from the little I know of them.

Historical linguists would disagree. The problem here is (European) Portuguese phonology, which is a big part of what makes intelligibility between Portuguese and Spanish so asymmetric. The written languages are quite close for the most part (though there are obviously vocabulary, such as Portuguese having borrowed quite a bit of French vocabulary and Spanish having greater Mozarabic influences, and morphosyntactic differences, such as the Spanish a-accusative), those are pretty surmountable. It's phonology that messes things up, which the heavy reduction of unstressed vowels in EP and the palatalisation seen in BP make Portuguese a lot more difficult for Spanish speakers to understand than the other way around.

I know some BP and ES (though I'd never call myself remotely fluent in either), but I find EP quite difficult to follow, FWIW. I wouldn't be at all surprised if the mutual intelligibility of BP and Spanish is higher than EP and Spanish.

This is a circular argument. English is one language because it is one language. Spanish and Portuguese are two languages because they are two languages.

No what the commenter was saying was that there are some pretty obvious differences between Spanish and Portuguese (not even remotely comparable to the differences between the mainline variants of English). See also:

https://en.wikipedia.org/wiki/Mutual_intelligibility

That's not what the commenter was saying; he was saying that the different dialects of English are one language because they're all English (one language). This is ultimately circular. I agree with his conclusion, but the distinction between language and dialect is a hazy one and his answer shined no light.

Well, having re-read their sentence... it definitely doesn't read that way to me.

But you can read it that way, if you want to.

No. English is one language because the entire world considers it to be one language. Spanish and Portuguese are two languages because the entire world considers it to be two languages.

That's still circular...

No it's not.

What defines a language? In the absence of context, I can't give you criteria for where the line is between a language or a dialect. But that doesn't matter, because the most important definition for "what's a language vs a dialect" is "how is it perceived by the world?". If the entire world thinks that Spanish and Portuguese are different languages, then they're different languages. It's that simple.

This isn't circular because I'm not saying "they're different languages because they're different languages". I'm saying "they're different languages because people say they're different languages".

It's circular because people say they're different languages because people say they're different languages.