> It's pretty amazing that Arabic, technically spoken in a contiguous region from Morocco to Saudi Arabia isn't mutually intelligible with itself, whereas English, French or Spanish, separated by oceans for centuries, even if they grew different variants, stay intelligible.

Language is political.

If a different political situation had come about you could see French, Spanish, Italian and Romanian dubbed one language named Neo-Latin, this language wouldn't be very mutually intelligible either

The Quran and its language ("Modern Standard Arabic") may be what binds the "dialects" together under one umbrella label "Arabic". Chinese is in a similar situation, whose writing system unifies the mutually un-intelligible "dialects" like Mandarin, Hokkien, and Cantonese as one "language" called "Chinese".

Such is an advantage of separating written and spoken forms of a language: as the spoken form inevitably mutates over time and fractures over space, having a stable written form means people who would otherwise consider each other foreigners may (1) share a common understandable set of historical and cultural documents, (2) communicate with each other at least in letters, (3) identify each other's language as but a "variation" of the standard written form.

English, French, Spanish, and Portuguese remain largely uniform across the Oceans because they are spread across the Oceans only recently, that is, not long before centralised nation states, compulsory education, sound recording, radio broadcasting, and YouTube.

The parallel to post-Roman Europe is interesting, where Latin lived on for quite a while as the language of writing. It gradually gave way to written forms of the various spoken languages, derivatives of Latin or others. During this time of parallel usage Latin, just by being there as an alternative for the elites to show off pedantry, may have helped establishing a tradition of keeping the new written languages continuously aligned with their permanently changing spoken counterparts.

Keeping Latin while it was helpful as a shared language when the scholar density was low and then moving on once local intellectual networks became stronger and mass education required efficiency may have been one of those "guns germs and steel" factors where Europe just happened to be lucky.

Your anecdote is also mentioned in the article [0] linked somewhere in this thread:

"The one thing not mentioned in the article is the idea of registers, which is the vertical transition between intermediate stages between full dialect and full MSA depending on the socio-lingiustic context of a conversation or other language production opportunity."

[0] http://languagelog.ldc.upenn.edu/nll/?p=40379

Would it be comparable to, say, Dutch and Afrikaans? They sound similar and up to a point people from those countries can have a conversation in their respective languages, but there's just so many differences that it's easier to switch to English.

Dutch and Afrikaans is probably in the top three most similar language pairs among Indo-European languages along with Spanish-Portuguese-Italian (a three way) and Danish-Norwegian bokmål. The fact that people switch to English is more a reflection of how good English language education is than the difference between the languages. If you are in Italy and you speak Spanish and English you’ll get more benefit from your Spanish outside tourist traps.

The spreading of English and Spanish across oceans is much younger than the spreading of Arabic. Arabic had many centuries more to diverge than English or Spanish. The regional separation of Arabic is only few centuries younger than that of Latin if we pretend for simplicity that the empire somehow had uniform language.

French and Spanish aren’t mutually intelligible with each other, despite both being dialects of Latin.

That’s a better analogy.

There’s no scientific reason to say all the Arabic dialects are “the same language” whereas all the modern Latin dialects are “different languages”.

Well try going from south of England to the north of Scotland and tell me these people speak the same language.

Concur. I simply cannot understand the English spoken by Scots.

Up till about 100 years ago, most of today's Arabic speaking areas were at least nominally under Turkish suzeranity, Turkey also was the centre of the Islamic world for so long, I wonder if the Turks did not try to replace Arabic with Turkish?

I believe they were fairly light handed in that regard, and respected the cultural independence of their people. Grain of salt though, vague memory.

The fact that the Tunisian dialect is infused with French words doesn't help with communication either, especially with someone from the middle East.