Sater Frisian
The last remaining living remnant of Old East Frisian is an Ems Frisian dialect called Sater Frisian or Saterlandic (its native name being Seeltersk), which is spoken in the Saterland area in the former State of Oldenburg, to the south of East Frisia proper. Saterland (Seelterlound in the local language), which is believed to have been colonised by Frisians from East Frisia in the eleventh century, was for a long time surrounded by impassable moors. This, together with the fact that Sater Frisian always had a status superior to Low German among the inhabitants of the area, accounts for the preservation of the language throughout the centuries.
Another important factor might be that after the Thirty Years' War, Saterland became part of the bishopric of Münster. As a consequence, it was brought back to the Catholic Church, resulting in isolation from the principal Protestant part of East-Frisia since about 1630, so marriages were no longer contracted with people from the north.
Read more about this topic: Saterland Frisian Language