Thanks for submitting the interesting popular article on a perennial topic of discussion. The subheading of the article begins with "Britain and the US share a common language – but English is spoken and spelled very differently on each side of the Atlantic." Spelling, first of all, is largely irrelevant, because native speakers of English all over the world often coped without standardized spellings and sometimes still do, and anyway can usually read one another's spellings with understanding. How "very different" the speech of the United States is from the speech of Britain is a matter of distinguishing how different is different enough to be a problem.

I read the other comments here before typing out this comment. As several of the comments say, distinguishing different varieties of speech as "dialects" rather than "languages" is often a matter of politics rather than a matter of linguistics. It happens that I was one of the Wikipedians who was active in updating the Wikipedia article "English language"[1] earlier this year so that it is now a "good article" by Wikipedia's article rating criteria. For years, there were all kinds of stupid edit wars on that article by editors who didn't bother to look up or read any sources, but when several Wikipedians agreed to look up sources together and check what the sources actually say, we reached consensus about how to improve the article. The main point about the English language is that it has a very large speech community with high mutual comprehensibility spread all over the globe. The spread around the globe came first from trade, migration, and colonization, but even after the British Empire dissolved, the spread of English has been maintained by telecommunications, travel, broadcasting, film, book publishing, study abroad, and the efforts of many national governments of non-English-speaking countries to promote knowledge of English through formal schooling and government administration. The majority of people who use English day-by-day now are not descendants of English settlers who live in the "inner circle" of English-speaking countries. When we consider that railroads, the telegraph, the telephone, voice radio broadcasting, passenger airplanes, and talking motion pictures were all invented in English-speaking countries, and were used in international communication as early for United States-to-Britain communication as for international communication between any other country pairs, it is not surprising that English has stayed remarkably homogeneous across the vast territory of the United States and has even stayed mutually comprehensible despite the political separation of the United States and Britain. Like the majority of my ancestors (I have only a little English ancestry, from which I gain my family name), most Americans are descended mostly from people who did NOT speak English when they arrived in North America, but who learned English to communicate with one another as residents of the United States. My two maternal grandparents were both born in Great Plains states of the United States, but their schooling (only primary schooling) was conducted entirely in the German language, and they learned English as a second language as native-born United States citizens to interact with neighbors.

There were earlier comments in this thread about Chinese. Absolutely, positively the different Sinitic languages are distinct languages, not mutually comprehensible, and it does violence to the English usage of the term "dialect" to refer to Mandarin and to Cantonese as "dialects" of Chinese. I speak Modern Standard Chinese fluently and have worked for many years as a Chinese-English interpreter. I have also studied Taiwanese, Cantonese, and Hakka (listed in decreasing order of proficiency). Mandarin and Cantonese are more distinct, in several respects, than English is from German or than French is from Spanish. Calling both Mandarin and Cantonese "dialects of Chinese" is simply a matter of the politics of China denying linguistic reality.

The distinctions among Sinitic languages as distinct languages apply even if you write them down in Chinese characters. Chinese characters still represent speech (not ideas or abstract concepts) and do so in a way that is specific to the particular Chinese (Sinitic) language that one speaks. The long story about this can be found in the books The Chinese Language: Fact and Fantasy[2] or Visible Speech: The Diverse Oneness of Writing Systems[3] by the late John DeFrancis or the book Ideogram: Chinese Characters and the Myth of Disembodied Meaning[4] by J. Marshall Unger. The book Reading in the Brain: The Science and Evolution of a Human Invention[5] by Stanislas Dehaene is a very good book about reading in general, and has a good cross-cultural perspective.

I'll give an example here of how Chinese characters reflect speech more than they reflect meaning-as-such. Many more examples are possible. How you might write the conversation

"Does he know how to speak Mandarin?

"No, he doesn't."

他會說普通話嗎？

他不會。

in Modern Standard Chinese characters contrasts with how you would write

"Does he know how to speak Cantonese?

"No, he doesn't."

佢識唔識講廣東話？

佢唔識。

in the Chinese characters used to write Cantonese. As will readily appear even to readers who don't know Chinese characters, many more words than "Mandarin" and "Cantonese" differ between those sentences in Chinese characters.

Many other examples of words, phrases, and whole sentences that are essentially unreadable to persons who have learned only Modern Standard Chinese can be found in texts produced in Chinese characters by speakers of other Sinitic languages ("Chinese dialects"). Similar considerations apply to Japanese, which is not even a language cognate with Chinese, and also links Chinese characters to particular speech morphemes (whether etymologically Japanese or Sino-Japanese) rather than with abstract concepts.

[1] https://en.wikipedia.org/wiki/English_language

[2] http://www.amazon.com/Chinese-Language-Fantasy-John-DeFranci...

[3] http://www.amazon.com/Visible-Speech-Asian-Interactions-Comp...

[4] http://www.amazon.com/Ideogram-Chinese-Characters-Disembodie...

[5] http://readinginthebrain.pagesperso-orange.fr/intro.htm