The UK entered WW2 on 3 September, 1939. Eighty years after the war's end, few people know that a British island's unique language was used as a clandestine code during Nazi occupation

"Bonsouair! Seyiz les beinv'nus!" announced Jo Thorpe, as she stepped up to the microphone and spread her arms in welcome. "Or for anyone who's not from Jersey, good evening and welcome! We're thrilled to see you tonight, and we hope you enjoy the show."

The lights dimmed, and her band, Sonneux, started playing. Accompanied by violin, double bass, guitar and recorder, Thorpe began to sing a familiar folk melody – only there was something surprising about her tune. Though it sounded like she was singing in French, in fact, she was singing in another language entirely: Jèrriais, Jersey's native language.

"In Jersey, we call this a veil'ye," she explained during a mid-show break, handing me a bowl of bean crock, a traditional, hearty island stew. "It's an evening of singing, music and storytelling, where people read, perform and share memories. Before radio and TV, a veil'ye was how people socialised. Every village had them. They were part of life. But the tradition died out, so we decided to bring them back. And of course, we couldn't do that without Jèrriais."

Jersey is the largest of the Channel Islands, an archipelago of six inhabited islands (and many more uninhabited ones) scattered over the English Channel, just over 14 miles from the French coast. Despite its proximity to France, it's actually a British Crown Dependency, and as such, the island has two official languages: English and French. But it also has Jèrriais.

Linguistically, Jèrriais's closest relative is Norman French, a dialect that dates back to the days when Normandy was still its own independent kingdom, and which incorporates many words from Old Norse, a legacy of the Normans' own Viking ancestry. Jèrriais shares much in common with the other languages of the Channel Islands, including Guernésiais (Guernsey) and Sercquiais (Sark), which are still spoken by a handful of people, and Auregnais (Alderney), which died out in the late 19th Century.

The first written record of Jèrriais dates from the 12th Century and the poet Wace, whose works Roman de Brut and Roman de Rou represent the earliest extant examples of the language – although Jèrriais was undoubtedly being spoken on the island long before Wace first put quill to parchment. As recently as the 1930s, Jèrriais remained the mother tongue for most Jersey-born people, but not everyone spoke quite the same version of the language. Since roads were poor and most islanders rarely travelled beyond their own home parish, every area of the island developed its own words, phrases and accents, which were often entirely different from those of their neighbours – an extraordinary fact on an island that measures just nine miles by five.

Today, however, it's a different story. According to the Endangered Languages Project, Jèrriais is a language on the brink. In the 2002 census, only 15% of islanders said they understood at least some Jèrriais words; today it's thought that figure is closer to 5%. Fluent speakers are even rarer; there are now fewer than 500 native Jèrriais speakers left, mostly in their 70s and 80s.

Among them is Francois Le Maistre. Now 87, he grew up in the........

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