There is no precise and official way to identify something as a "language" rather than a "dialect".
For example: there are five major dialects of Hindi, some of which are mutually unintelligible -- yet few people call them separate languages. On the other hand, standard Hindi is mutually intelligible with Urdu -- they use different writing systems, but as spoken languages are roughly as close as American and British English -- yet they are considered different languages. Similarly, China has a vast diversity of mutually unintelligible languages -- or, rather, what outsiders call "languages", and what the Chinese government insists are merely dialects of a single language: "Chinese".
What's going on here? Max Weinreich's quip that "a language is a dialect with an army and a navy" seems to hit the mark. If Britain and America had not become allies in the late 19th century, then the ongoing antipathy between them would have required their speaking systems to be classified as "languages". But that would have been a political necessity, not a linguistic one.
As an aside -- as somebody who grew up in America and emigrated at age 31 to Britain -- I must say that the oft-cited differences in spelling and pronunciation are the least important areas of divergence. Much more significant (and confusing) are the idiomatic divergences. For example: in the UK, people commonly say "are you alright?" as a greeting, equivalent to the American "how do you do?" or "what's up?". But in American, "are you alright?" is an expression of serious concern -- something you might say to somebody walking around in a state of confusion with an apparent head injury. It was extremely disconcerting when people kept asking me this -- I thought I must be acting/looking really strange to provoke such continual concern!
To your "dialect with an army and a navy" point, don't forget that in the US a language is taught called "Serbo-Croatian." In Serbia, it's "Serbian" and is its own language. In Croatia, it's "Croatian" and it is also a separate language locally despite being almost entirely mutually intelligible with speakers of the Serbian form.
(To add to the fun, in the UN it's called "Bosnian-Croatian-Montenegrin-Serbian", but who's counting?)
My wife studied this language once. The department she was studying it in called it "BCS" (for Bosnian-Croatian-Serbian); she eventually just took to calling it "Bosnifuckit".
Absolutely, a great example! I was just on holiday in Sarajevo (a lovely place -- I'd highly recommend going there before they resume shooting at each other). It's full of insignificant little differences which really drive this point home. Where the UK/US split can create a figurative minefield of idiomatic confusion, the Bosnian/Serbian/Croatian split has created literal minefields. Annoying!
In the Chinese case I read there are political implications for using "language" vs "dialect". Chinese is a very diverse country (geographically, ethnically, linguistically) and keeping the whole country unified is the role of the government. Thus this one use the terms dialect which implies all this languages are of in fact one with little differences rather than language that implies big differences. Even the name of the standard language is subject to connotation with 7 different words for it.
And this trick is also played in France (my country). Alsatian is called a dialect (insinuating a dialect of French) whereas is it linguistically related to German; and indeed it doesn't have a fixed written form and Standard German (Hochdeutsch) orthography is often used for it. Same goes for Flemish, etc.
While now France's dialect policy is a complete turnabout of his old one (repression), the dialect word is still used in place it shouldn't technically be.
I remember seeing a documentary of US origin which had interviews with Scots that were subtitled, despite the fact that they were speaking English (and as an Australian, perfectly intelligible to me).
So I took that as some evidence that not all English dialects are mutually intelligible.
The show Trawlermen which was shown in the UK was subtitled for native audiences since the accents on some of the men were so thick & unintelligible (at least to my friends' & my own ears in S England).
The first time I came to the UK, I remember sitting across from some rather loud Scots on the tube. I think they were talking about work; I wasn't sure, because the only word I could clearly understand was "fookin'" (an adjective which can apparently mean almost anything). Definitely not mutually intelligible to my California-trained ears.
There are always people with varying degrees of dialect usage.
This fellow[0] is quite, but not totally unintelligible to my English-as-a-second-language ears and there are Scottish where you recognize their origin in the first sentence but still understand them 100%.
Same with Bavaria, really. Most of us from Munich have a very light dialect that's really not that difficult compared to many people from the countryside.
edit: The Bavarian dialect also has quite a few grammatical quirks, whereas the Saxons just pronounce German very different, but the grammar is very standard.
For example: there are five major dialects of Hindi, some of which are mutually unintelligible -- yet few people call them separate languages. On the other hand, standard Hindi is mutually intelligible with Urdu -- they use different writing systems, but as spoken languages are roughly as close as American and British English -- yet they are considered different languages. Similarly, China has a vast diversity of mutually unintelligible languages -- or, rather, what outsiders call "languages", and what the Chinese government insists are merely dialects of a single language: "Chinese".
What's going on here? Max Weinreich's quip that "a language is a dialect with an army and a navy" seems to hit the mark. If Britain and America had not become allies in the late 19th century, then the ongoing antipathy between them would have required their speaking systems to be classified as "languages". But that would have been a political necessity, not a linguistic one.
As an aside -- as somebody who grew up in America and emigrated at age 31 to Britain -- I must say that the oft-cited differences in spelling and pronunciation are the least important areas of divergence. Much more significant (and confusing) are the idiomatic divergences. For example: in the UK, people commonly say "are you alright?" as a greeting, equivalent to the American "how do you do?" or "what's up?". But in American, "are you alright?" is an expression of serious concern -- something you might say to somebody walking around in a state of confusion with an apparent head injury. It was extremely disconcerting when people kept asking me this -- I thought I must be acting/looking really strange to provoke such continual concern!