Two things strike a casual visitor to the post office at Skigersta, and the first is its size. With its pebble-dash walls and shiny slate roof, it resembles almost every other building here at the northern tip of the Isle of Lewis.

But for all its solidity, and notwithstanding the official Post Office sign that gleams like a red beacon in the pale green landscape, it has all the grandeur of a Wendy house. Through the little door, flanked by a pair of little windows, is a single room, the two main features of which are a standard-issue post office counter and a display of greetings cards.

But, once inside, what strikes the visitor has nothing to do with scale. It is language. Listen as postmistress Margaret Macleod flips through the pension books, and the chances are you will hear little said that isn’t in Gaelic. There’s Gaelic gossip and Gaelic small talk, and even the business is transacted in Gaelic. It’s sweet and lilting, this old Celtic tongue, and it can name Welsh, Breton and Cornish among its cousins. The Scottish version (pronounced “gallic”) is so close to its Irish sister (pronounced “gaylik”) that a speaker of one can understand a speaker of the other, and the same could once be said of Manx.

But while older residents cling to Gaelic, so that an hour may pass in Margaret’s post office with little else being spoken, each and every one of the greetings cards on the display rack is written in English. “Happy Anniversary!” they shout, and “Well done! You passed your exams!”. But as another customer enters the tiny office, the greeting of choice will like as not be “Madainn mhath”.

Perhaps nothing more eloquently sums up the plight of an endangered language than this disjunction between old and new. So it is fitting that few people are more saddened by this state of affairs, or feel more keenly that the traditional life is slipping away, than Margaret’s daughter, Jayne.

Browse the tourist guides to the Scottish highlands and islands and you might wonder if things are really so gloomy. Since the establishment of Comhairle nan Eilean (the Western Isles council) in 1975, Gaelic has been encouraged through the education system. Parents can elect to send their children to a school where every subject, including maths and science, will be taught in the Gaelic medium, and in recent years there has been talk of a renaissance in Gaelic culture, with new learners supposedly reversing the decline in speakers.

But Jayne, who was brought up in this enchanted country, and who knows the community as well as she knows the windswept clifftops with their ruined bothies and ancient stone circles, shares little of this optimism. “Gaelic is definitely dying out,” she says, and as evidence she cites the attitude of her peers. “It seems to have an image problem among the youth, and it’s considered uncool and old-fashioned to speak it.”

For a 19-year-old, Jayne is exceptional. When most people of her age were looking to complete their education in Glasgow or Edinburgh, she chose to remain on Lewis and to study for a BA in Gaelic language and culture at a college in the island’s main centre of population, Stornoway. In her spare time, she organises traditional dancing classes at the Paigh Dhonnchaidh arts centre along the road from her mother’s post office. She has taken part in a project to record the memoirs of older residents, and she even hosts a CD review programme on the local radio station, nan Gaidheal.

In all of this, she says, her inspiration is her grandmother, who lives with Jayne and her parents, and whose wealth of memories and stories endlessly fascinates and motivates her. This close contact with the past has made Jayne realise just how much will be lost if Gaelic dies. “It is so expressive,” she says. “There are sayings and phrases that just can’t be translated into English. But it’s not just that. The language and the culture go hand in hand, and it makes me sad to see them slipping away.

It’s part of my roots - part of my ancestry - and if I did ever move to Glasgow or somewhere, I would want to know that it was all still here.”

It’s the irrevocability of such loss - the fact that once languages and their attendant cultures have ceased to live in the community, no amount of resurrection through textbooks and scholarship can ever restore their vitality - that has inspired Unesco to designate February 21 each year as International Mother Language Day.

Like Jayne, Unesco measures the degree to which a language is under threat by looking at trends among the young. If 30 per cent of children in a community no longer learn a language, experts generally reckon it to be endangered. And, according to Unesco’s most recent estimate, around 3,000 languages - that’s half the number spoken in the world today - are in peril.

Does it really matter if the world no longer hears a language formerly spoken in a small corner of the British Isles - or, for that matter, on the Andaman Island in the Gulf of Bengal, where only a few dozen people now speak Onge and Shompen? Unesco’s director-general, Koichiro Matsuura, believes it does. “Each is a unique response to the human condition,” he says, “and each is a living heritage we should cherish.”

If ever a group of people was destined to have a unique response to the human condition, it was the Sami, erstwhile nomads who for thousands of years followed the reindeer herds on their annual migration in and around the Arctic Circle, or fished the icy waters of the Barents Sea.

Today, around 80,000 Sami live in the extreme north of Europe, where their former herding grounds - an arc of territory delineated not by human agency but by reindeer instinct - are now divided arbitrarily between the modern nation states of Norway, Sweden, Finland and Russia.

After centuries of oppression, first by the Christian priests who came to collect taxes and to stamp out shamanism, and more recently by Stalin, who murdered their leaders and corralled the nomads into collective farms, the Sami are now resisting what some see as inevitable cultural and economic obliteration. They send delegations to conferences of indigenous peoples around the world (“We are privileged as indigenous people because nobody is actually killing us at the moment,” one activist comments wryly), oppose industrial despoliation of their hunting grounds, and campaign against the sometimes highly discriminatory policies of the various nation states in which they find themselves (as recently as 1992, the Swedish legislative assembly ruled against protecting reindeer hunting grounds, and the Sami see it as significant that Sweden has yet to ratify an international convention on the rights of indigenous and tribal peoples).

But just as important as the political and legal battles is the war on shame. For it’s an astonishing fact of life in modern, progressive Scandinavia that many Sami (they reject the Finnish terms “Lap” and “Lapland”, just as the Inuit threw off “Eskimo”) now deny their ancestry, hiding it from their children lest it hinder their progress in the modern world.

It’s a phenomenon that appals 18-year-old Anna Karrstedt. She is at high school, and lives with her family in the Kiruna region of Sweden, an area within the Arctic Circle where many Sami communities work and where the Sami parliament, essentially a counselling body, held its first session in 1993.

“Lots of my friends are Sami but don’t say they are because they are ashamed of it,” says Anna. “Some weren’t even told by their parents until a couple of years ago, and now they are pushing it away. I’m lucky. My parents have always told me, ‘You are Sami’, and it’s the greatest thing that ever happened to me.”

Like Jayne Macleod on Lewis, Anna’s enthusiasm for her cultural heritage is inspired and nourished by close contact with her grandmother, Ellen-Maria, who has taught her many of the old skills associated with reindeer tending.

And where Jayne regards Gaelic as her first language, so Anna regards Sami as her mother tongue.

Like Gaelic, Sami comes in a variety of forms, all of which have developed from a common root over the past 1,000 years. Today, fewer than half of Sami speak the language - hardly surprising given that its use was banned in schools from the end of the 19th century until the 1950s. But Anna speaks it when she is with her family, and sings it through the traditional Yoik, a Sami song form with a melody and structure as different from the music of the rest of Scandinavia as the characteristic high-cheekboned Sami face is from that of most Scandinavians.

“I get so much joy out of being Sami,” she says. “I feel my Sami life is like an extra life. I have my Swedish life and my Swedish friends, but at the weekends I go off to the mountains with my uncle and get to be with the reindeer and with nature. It’s like a bonus life, and I know this is the way I want to live.”

Anna’s grandmother has mixed feelings about the onward march of progress.

Having spent many of her 77 years on the move, living in tepees and turf houses, with only birch log fires for comfort while temperatures outside dropped to minus 50C during the long, dark winters, she says she is “happy - very happy” to be living in the relative comfort of a modern Sami village. “When you live like this and compare it with how you lived before, it makes you content,” she says.

But Ellen-Maria, too, is saddened to see how the Sami language is becoming studied and learned from books, and can’t understand why so many modern parents, while continuing to speak Sami between themselves, insist on using only Swedish with their children. “We taught our children some Swedish so they could manage in school,” she says. “But we taught them proper Sami that came from the heart and from the inside.”

It remains to be seen whether Sami can survive as a living language when the conditions to which it was once such a unique response are changing beyond recognition. For in the same way that young people in the Hebrides see little future in crofting or crab fishing, few young Sami see themselves making a living out of reindeer.

Today, herders must produce more and more meat to make a living, and that meat must be processed by large and highly regulated abattoirs. Yet, at the same time, grazing land is under pressure from logging, mining and the leisure industry. Anna’s father, who works in the local tourism office educating visitors about Sami customs, sees the language dying with the lifestyle. And he gives as an example the Sami words for snow.

“Sami has hundreds of words for snow,” he says. “There are many kinds of dry snow and even more kinds of wet snow. The reindeer feed on mosses, and people needed to know what sort of snow was on the ground so they could graze their herds. But if you don’t have these traditions any more, why should you have the language?”

But like Jayne Macleod, who is determined to pass on the torch of Gaelic culture, his daughter feels duty-bound to learn what she can and to pass on what she knows. “If there aren’t any reindeer when my grandchildren are born, I would at least like to tell them how it was for me and for my grandmother,” says Anna. “And, most of all, I would like to pass on the language.”

To tie in with Mother Language Day, Discovery Channel is showing Archives of Babel (Friday, February 21, 9pm). By analysing ‘markers’ embedded in today’s languages - the traces of Celtic, Latin and Norse in modern English, for example - linguist Donald Stilo outlines his quest to pinpoint a time and place when all people could communicate with one another.The channel has also teamed up with Unesco to produce several two-minute vignettes to be shown throughout the day. These will highlight threatened languages and cultures around the world, and include footage of Jayne Macleod in Lewis and Anna Karrstedt in Swedenl Atlas of the World’s Languages in Danger of Disappearing, edited by Stephen A Wurm with maps by cartographer Ian Heyward, is published by Unesco Publishing, pound;13.25