"by the sentence from the Polish wikipedia 
 I am awfully sorry but Polish wiki is very poorly done.And it certainly doesn't included much of information found on English wiki.Forster's article for example doesn't included much of information from Lukasiewicz books.As to Forster's hatred for Poles and his belief they are animals-I think it needs to be added, since It led to such horrible things as German politicians saying Poles are wolves to be shot etc. Since you added East German relation to Forster-it intersting that his prejudice against Poland was reflected in the same prejudice about Polish economy in campaign made by East German authorities.