Low German

From Wikipedia:

Low German or Low Saxon (Plattdüütsch, Nedderdüütsch, Nedersaksisch; Standard German Plattdeutsch, Niederdeutsch; Dutch Nedersaksisch — see Nomenclature) is any of the regional language varieties of the West Germanic languages spoken mainly in northern Germany and the eastern part of the Netherlands.

Dialects of Low German are widely spoken in the northeastern area of the Netherlands (Dutch Low Saxon) and are written there with Dutch orthography.

Variants of Low German were widely (and are still to a far lesser extent) spoken in most parts of Northern Germany, for instance in the states of Lower Saxony, North Rhine-Westphalia, Hamburg, Bremen, Schleswig-Holstein, Mecklenburg-Vorpommern, Saxony-Anhalt and Brandenburg. Small portions of northern Hesse and northern Thuringia are traditionally Low Saxon speaking too. Historically, Low German was also spoken in formerly German parts of Poland as well as in East Prussia and the Baltic States of Estonia and Latvia. The language was also formerly spoken in the outer areas of what is now the city state of Berlin but in the course of urbanization and national centralization in that city the language vanished. (The Berlin dialect itself is a northern outpost of High German and typologically a Missingsch variety, although rarely recognized as the latter).

Today, there are still speakers outside of Germany and The Netherlands to be found in the coastal areas of present Poland (minority of ethnic German Pommersch speakers who were not expelled from Pomerania, as well as the regions around Braunsberg). In the Southern Jutland region of Denmark there may still be some Low German speakers in some German minority communities, but the Low German and North Frisian dialects of Denmark can be considered moribund at this time.

Wikipedia: Low German (English)
Wikipedia: Niederdeutsche Sprache (German)
Wikipedia: Plattdüütsch (Platt)