The population declined after the late 14th century because of an agricultural crisis, later intensified by the Hussite wars. Rural settlements were abandoned and cities lost a part of their population. This caused migration which led to mixing of Germans and Slavs. The Silesian minority soon adopted the language of the German majority. Most Polish linguistic enclaves in the south of Lower and Middle Silesia disappeared; these regions became largely German. In the western part of Silesia the Polish language survived only in the region around Zielona Góra (Grünberg) and Otyń (Deutsch Wartenberg) and in the agricultural plain to the left of the Oder in a triangle between Wrocław (Breslau), Kąty Wrocławskie (Kanth), Strzelin (Strehlen) and Oława (Ohlau). Almost all German linguistic enclaves in Upper Silesia had vanished in the 16th century. Only the towns of Opava (Troppau), Kietrz (Katscher) and Bielsko (Bielitz) remained largely German. This process was moreover encouraged by the often use of Czech as the official language in Upper SIlesia at that time, as both languages were still closely related.