How much are Poles (Slavs) themselves guilty for allowing itself to fall as victims of Germanization, in such a vast numbers. Little of self-criticism can`t be harmful. Question is interesting, for yesterday Germanization, tomorrow Anglicanization. Yes? So, what is the genesis of the problem?
This is indeed an interesting questions. First, I don't think that Poland is in any way threatened by some sort of anglicization now or in the coming 500 years. To gain some good insight into the processes of anglicization, it would be better to ask why people such the Irish or the Scotish let English be imposed on most of them as their native language. Why the vast majority of them do not use the language of their Celtic ancestors at home now? What made them give up Scottish or Irish and adopt English in their place?
As for the germanization of the Polish people, we know quite a lot about why and how it happened in Silesia. And I mean today's Lower Silesia here since the province being purely Slavic in the Middle Ages has become ethnicly German later on. In year 1097 Wrocław (Breslau) - according to Gallus Anonymus, the most important maedival chronicler of Poland - was one the three main capitals (or seats - sedes regni principales) of the kingdom:
([says Ladislaus Herman about his son Boleslaus the Wrymouth]: Bolezlaus vero, legitimus filius meus, in Wratislaw et in Cracovia et in Sandomir sedes regni principales obtineat
The crucial figure who (uintentionally) laid the foundations for the prospective germanization of Silesia was another Piast ruler, Henry the Bearded (1165 - 1238), great grandson of Boleslaus the Wrymouth, who brought numerous agricultural settlers from Germany (but also from Flanders, Frisland and Holland) to Lower Silesia, granting them land and exempting them from tax for a certain period of time.
One of the major factors behind the subsequent ethnicity change in Lower Silesia was that those German-speaking settlers were intentionally separated for economic reasons from the indogenous Polish population of the region. Thus - rather than mixing with the locals and subsequently adopting Polish as their first language in the following generations - these people who were commonly referred to as "guests" in the documents throughout the whole 13th century, became majority on the left bank of the river Oder in Lower Silesia. The land on the right bank of the river, however, remained Polish for a much longer time being accordingly described as the "Polish bank" of the river until, I think, the18th century or so.