Frisian is a language spoken by a minority of people in the northern part of the Netherlands, mostly in the province of Friesland (Fryslân in Frisian). There are around 440,000 speakers there, plus about 10,000 more in East-Friesland (in Germany) and North-Friesland in Germany up to the island of Sylt and including the island of Roem (in Denmark). It is most similar to Old English (but not Modern English), the Netherlandic language, and LowGerman.
The earliest definite written examples of Frisian are from approximately the 9th century AD. A few examples of Runic inscriptions from the region are probably older and possibly in the Frisian language.
Old English is arguably more closely related to Frisian than any other language; even modern English bears some resemblance if one ignores the French vocabulary grafted on to English during the Middle Ages. A rhyming couplet demonstrating this is in common circulation:
Good butter and good cheese
Is good English and good Fris
The first line of this verse is pronounced essentially identically in both languages.
See also Common phrases in different languages.