I'm reading a book called Peasants in the Middle Ages by Werner Rosener, and he says about the legal rights of settlers to eastern Germany: "in some places there was even explicit mention of a ius flandricum". Neither Google or a JSTOR search tells me what that means.
This probably refers to the rights of the Flemish and Dutch settlers that were invited to help reclaiming wetlands in northern Germany and other places in the 12th century and onwards. They had their own settlements, with permission to retain their own laws and entitlements ('ius flandricum') from home.
The 1938 book "Voortrekkers van Oud-Nederland" (pioneers of old-Netherlands) by F. Dekker [1] contains seven chapters about them. There is a lengthy summary available on the web at http://home.planet.nl/~artrako/Historie/NedKoloniesDld-NL.html [Dutch] ( Google translation to English )
It says that "The presence of Dutch colonies can be deduced not only from charters, but also from special customs and rights, which have existed for centuries for a part of the population in some of the aforementioned regions.", which is what your question is about.
The summary specifically mentions the Flemish (Fläminger) rights and customs. There were several such settlements. In one such place they were exempt from tax until the end of the 18th century. Other laws differed too from their surrounding regions.
When a Flemish researcher visited the area as late as 1860 he noted that they still had a lifestyle and a dialect that was different from other (German) inhabitants. In fact the Flemish visitor was easier to understand by them than the local Germans.
The summary of that book doesn't mention 'ius flandricum' literally. It does contain however much more information other than given above.
- [1]: F. Dekker: "Voortrekkers van Oud Nederland", Boucher publisher, Den Haag / The Hague; 1938.
The web article refers to these additional sources:
-- Emile de Borchgrave; Histoire des colonies belges, qui s'établirent en Allemagne pendant le douzieme et le treizième siècle; 1865.
-- Pastor Bölke; Geschichte eines Flämingdorfes.
-- F. Dekker; Voortrekkers van Oud Nederland; Uitgeverij Boucher te Den Haag; 1938.
-- Hoche; Historische Untersuchungen über die Niederländischen Kolonien in Niederdeutschland, Halle 1791.
-- Dr. Theodor Rudolph; Die Niederländischen Kolonien in der Altmark
-- Richard Schröder; Die niederländischen Koloniën in Nord-Deutschland; 1880.
-- A. von Wersebe; Ueber die Niederländischen Colonien, welche im nördlichen Teutschlande im zwölften Jahrhundert gestiftet worden sind; 1815.