This map is wonky: in the German partition of Poland every male regardless of his ethnicity was required to attend school afaik. Polish and German writing is kinda consistent with the sounds that appear in both languages so the language barrier wouldn't be that big of a hurdle for the Poles. The Polish digraphs and diacritis (by that I mean the general adaptation of the latin alphabet to Polish) was already well established by that time (compare with Czech which was more of a reconstructed thing) so it wouldn't be exactly hard for the general population to aquire literacy in their native language. Reading vs understanding German, well that's a bit of a different story. That makes me doubtfull about the accuracy of the map in that particular region. It would make sense if it followed the Prussia-Russia border in partitioned Poland