If you look it up, you'll generally find that the language that's most closely related to English is Frisian, which is actually three closely related languages: West Frisian in the Netherlands, North Frisian on the Schleswig-Holstein peninsula and the offshore islands, and East Frisian (or Saterfrisian) in the Saterland in northern Germany. (The area called "East Frisia" no longer speaks Frisian, but Frisian-influenced Low German.)
But in fact, there is another language that's much closer to English but still separate: Scots. Not Scots Gaelic, which is closely related to Manx and Irish. And not Scottish English, which is a variety of English heavily influenced by Scots. But Scots itself, which is spoken in various dialects all over Scotland, in Northern Ireland, and in the Orkney Islands.
Like most minority languages, Scots went through a low point in the 20th century, and is now undergoing something of a revival: some of the Scottish Parliament's reports are written in it. Fortunately, it has had quite a lot of literature one way and another: I'm particularly fond of a horror story by Robert Louis Stevenson called "Thrawn Janet". That's a version written in modern standard Scots spelling: Stevenson's original, which is more English-adapted, is also available at the excellent website Scots-Online.org.
Anyhow, here's a bit of Scots that should be fairly easy going while still showing off the contrast between English and Scots: William Lorimer's New Testament in Scots (only the Devil speaks Standard English). Here's the first few verses of the Gospel of John:
IN THE BEGINNIN o aa things the Wurd wis there ense, an the Wurd bade wi God, an the Wurd wis God. He wis wi God I the beginnin, an aa things cam tae be throu him, an wiout him no ae thing cam tae be. Aathing at hes come tae be, he wis the life in it, an that life wis the licht o man; an ey the licht shines i the mirk, an the mirk downa slocken it nane.
There kythed a man sent frae God, at his name wis John. He cam for a witness, tae beir witness tae the licht, at aa men micht win tae faith throu him. He wisna the licht himsel; he cam tae beir witness tae the licht. The true licht, at enlichtens ilka man, wis een than comin intil the warld. He wis in the warld, an the warld hed come tae be throu him, but the warld miskent him. He cam tae the place at belanged him, an them at belanged him walcomed-him-na. But til aa sic as walcomed him he gae the pouer tae become childer o God; een tae them at pits faith in his name, an wis born, no o bluid or carnal desire o the will o man, but o God.
Sae the Wurd becam flesh an made his wonnin amang us, an we saw his glorie, sic glorie as belangs the ae an ane Son o the Faither, fu o grace an trowth. We hae John's witness til him: "This is him," he cried out loud, "at I spak o, whan I said, 'Him at is comin efter me is o heicher degree nor me, because he wis there afore iver I wis born'." Out o his fouth ilkane o us hes haen his skare, ay! grace upo` grace; for, atho` the Law wis gien throu Moses, grace an trowth hes come throu Jesus Christ. Nae man hes e'er seen God: but the ae an ane Son, at is God himsel, an liggs on the breist o the Faither, hes made him kent.
As the slogan says, "Scotland: A Twa-Leidit Folkrick" (a bilingual culture).