Mennonites are traditionally multilingual with which Low Prussian dialect of East Low German, with Dutch influence language was their first?
Russian Mennonite:  The rest were forcibly relocated, so that few of their descendants now live at the location of the original colonies. Plautdietsch language: Plautdietsch or Mennonite Low German, was originally a Low Prussian dialect of East Low German, with Dutch influence, that developed in the 16th and 17th centuries in the Vistula delta area of Royal Prussia.
Plautdietsch