Across the street from the building that houses the linguists, philosophers, and computer scientists at my university (on the left in the picture above) is the city’s huge central mosque. Hundreds of Muslims stream out of its doors after lunchtime prayers on Fridays to socialize in the courtyard. Men with men, of course (women have a separate exit door round the back).
A few yards away on the mosque side of the street is Maqbool’s, a grocery store run by Pakistanis (I’ll use that designation here, though many of the ethnic Pakistanis I interact with every day have Scottish accents, and some were born here).
Yesterday I stopped by Maqbool’s to buy fresh chili peppers (at about a fifth of the supermarket prices elsewhere!). Ahead of me at the cash desk was a young woman, chatting and laughing with the storekeeper in a language I had studied for a while at the University of York when I was an undergraduate: Hindustani (also known as Hindi-Urdu).
I watched with interest. Social interaction between the sexes in public is rather unusual in the local Muslim population. And then I noticed the woman’s long black hair flowing free around her shoulders. No nod toward standard Muslim head-covering at all, not even a loosely draped headscarf. That was unusual too. It occurred to me that she was probably Indian, not Pakistani.
As if reading my mind, the man behind the counter unexpectedly stopped speaking Hindustani and asked in English: “Are you from India?”
She said she was. There was a little more friendly Hindustani conversation, and she paid for her purchases and said a smiling goodbye.
The fact that at first he hadn’t noticed is a measure of the similarity between what Indians always call Hindi and Pakistanis always call Urdu. The two are often referred to, even by native speakers, as quite different languages — especially by those who are tempted to individuate languages by reference to writing systems (a practice that breeds much of the confusion over how many languages are spoken in China). A standardized variety of Hindustani known in India as Modern Standard Hindi is the designated official language of the Indian federal government; 
it is written (from left to right) with the Sanskrit-derived Devanagari script, and draws its learned and religious terminology from Sanskrit sources. And a standardized variety known as Modern Standard Urdu is the national language of Pakistan, written (right to left) with a script adapted from Arabic by the Persians, Nastaliq; it draws the learned and religious strata of its vocabulary from Persian and Arabic sources.
The religious, cultural, and orthographic differences were amplified in 1947 into a massive political separation when Britain partitioned South Asia into India and Pakistan. The process led to 14 million people becoming refugees. At least two million disappeared (the actual number killed in the violence as Hindus fled south and Muslims fled north has never been determined, but it was huge).
We have seen similar tragedies more recently elsewhere: The Serbs, Croats, and Bosnians of the former Yugoslavia use essentially the same language, but for cultural, religious, and above all political reasons they were reconstructed in the 1990s as distinct languages (see my June 2013 post, A Trinity of Languages), and the speakers were induced to fight disastrous and near-genocidal wars against each other.
Despite the disastrously hostile relations that have persisted between India and Pakistan (both nuclear-armed), language does not divide the populations of the Hindustani-speaking areas. Hindi and Urdu are not just recognizably linguistic cousins (like Hebrew and Arabic, or German and Dutch), but much closer — close enough to be called regional dialects of a single language. They share all of their basic structure, especially in grammar.
As long as it is just casual chat over the grocery counter, there is no communication barrier at all. And individual Indians and Pakistanis do not hate each other. Before 1947, Hindus and Muslims (and Buddhists and Christians) had coexisted fairly peacefully for centuries in South Asia.
Maqbool is a visibly Pakistani name (the letter q originates in Persian loanwords which Hindus generally don’t use); the young Indian woman could have noted that and shopped elsewhere.
The man behind the counter could have turned cold and unfriendly on realizing her nationality.
These things did not happen. Far away from the toxic political hostility and military posturing of the rival countries of India and Pakistan, they laughed and joked in a friendly way across the supposed barriers of language, culture, ethnicity, religion, dress, nationality, and sex.
Imagine, as John Lennon put it.