After the partition of poland in the late 18th century, Prussia aquired a large part of Polish territory. Those areas suffered heavy Germanization process through assimilation and suppression of Polish culture.
By the end of the 19th century, kids were instructed to read and write German language, and in German only. Polish was only used as a spoken language at home, and maybe with your neighbours, but definitely not in an interaction with anybody outside your village, anybody who would see their social rank rise had to be germanized.
The results is that people would only speak a Polish dialect with their family, but have absolutely no idea how the language is read or written, since the orthography system is very different from the German system.
I know there was some Polish newspapers, but who could read them?
Once Poland was restored in 1918, people certainly couldn't understand anything to the written language, could they?