Upper Silesia language used Polish
Bohemia, Austria and Prussia
In 1327 the Upper Silesian dukes, like most of their Lower Silesian cousins, had sworn allegiance to King John of Bohemia, thereby becoming vassals of the Bohemian kingdom. During the re-establishment of Poland under King Casimir III the Great, all Silesia was specifically excluded as non-Polish land by the 1335 Treaty of Trentschin becoming a land of the Bohemian Crown and — indirectly — of the Holy Roman Empire. By the mid-14th century, the influx of German settlers into Upper Silesia was stopped by the Black Death pandemic. Unlike in Lower Silesia, the Germanization process was halted; still a majority of the population spoke Polish and Silesian as their native language, often together with German (Silesian German) as a second language. In the southernmost areas, also Lach dialects were spoken. While Latin, Czech and German language were used as official languages in towns and cities, only in the 1550s (during the Protestant Reformation) did records with Polish names start to appear.
Upper Silesia was hit by the Hussite Wars and in 1469 was conquered by King Matthias Corvinus of Hungary, while the Duchies of Oświęcim and Zator fell back to the Polish Crown as a part of Lesser Poland. Upon the death of the Jagiellonian king Louis II in 1526, the Bohemian crown lands were inherited by the Austrian House of Habsburg. In the 16th century, large parts of Silesia had turned Protestant, promoted by reformers like Caspar Schwenckfeld. After the 1620 Battle of White Mountain, the Catholic Emperors of the Habsburg dynasty forcibly re-introduced Catholicism, led by the Jesuits.After the 1939 German invasion of Poland, Polish Silesia was annexed to the Nazi German Reich as part of the  Gau of Silesia. In 1941 Upper and Lower Silesia were split into separate Gauer.
After 1945, almost all of Upper Silesia that was not ceded to Poland in 1922 was placed under the administration of the Republic of Poland. German civilians, as well as Nazi criminals, were interned in labor camps such as the Zgoda labour camp. The majority of the German-speaking population that had not fled was expelled, an activity that was euphemized as "transfers [to] be effected in an orderly and humane manner" in accordance with the decision of the victorious Allied powers at their 1945 meeting at Potsdam. This expulsion program also included German-speaking inhabitants of Lower Silesia, eastern Brandenburg, eastern Pomerania, Gdańsk (Danzig), and East Prussia. The German expellees were transported to the present-day Germany (including the former East Germany), and Polish migrants, a sizeable part of whom were themselves expelleés from former Polish provinces taken over by the USSR in the east, settled in Upper Silesia. A good many German-speaking Upper Silesians were relocated in Bavaria. A small part of Upper Silesia stayed as part of Czechoslovakia as Czech Silesia.
The expulsions of German-speakers did not totally eliminate the presence of a population that considered itself German. In contrast to the situation in Lower Silesia, where almost the totality of the pre-war population that was expelled was exclusively German-speaking, the pre-war population of Upper Silesia was in considerable number Roman Catholic mixed bilingual that spoke both German and Polish dialects, and their Polish linguistic skills were considered solid enough for them to be kept in the area (Aleksandra Kunce, Being at Home in a Place: The Philosophy of Localness, pp. 41-112 [1]). Firstly, after the war, Poles displaced from Polish territories incorporated into the USSR settled in Upper Silesia, but also Polish settlers from other overpopulated parts of Poland. And later, in the years 1945-1989, a large number of Poles from various parts of Poland settled in Upper Silesia, who received work, e.g. in the mines.
The area formally became part of the Republic of Poland by virtue of the German-Polish border treaty of 14 November 1990. With the fall of communism and Poland's joining the European Union, there were enough of these remaining in Upper Silesia to allow for the recognition of the German minority in Poland by the Polish government.
language spoken official language linguistic usage dialect spoken vernacular employed