Where would you find the territory known in the local language as "Ellan Vannin"?

The "local language" referred to in the question title is Manx, one of the minor languages spoken in the British Isles. Manx is a Celtic language, one of six such languages still spoken in parts of northwest Europe, the others being Irish, Scots Gaelic, Welsh, Cornish and Breton (spoken in Brittany, France).

Manx is the old language of the Isle of Man, which is located in the Irish sea, roughly equidistant from the coasts of Ireland, England and Scotland. There has been a steady increase in the number of speakers of Manx in the last fifty years. Despite the dominance of the English language – as in all parts of the British Isles -- the Manx language has never fallen completely out of use, with a minority having some knowledge of it as a heritage language, and it is still an important part of the island's cultural heritage.

Manx is an interesting example of language revival efforts; in 2015, around 1,800 people had varying levels of second-language conversational ability. Since the late 20th century, Manx has become more visible on the island, with increased signage, radio broadcasts and a Manx-medium primary school. The revival of Manx has been made easier because the language was already well recorded.

The expression “Ellan Vannin” in the question means “Isle of Man.” This expression shows the links that Manx has with the Gaelic languages in the two neighbouring countries. Take the word for “island”:

Manx: “Ellan.”

Irish: “Oileán.”

Scots Gaelic: “Eilean."