Nisht: ZD clarified that he was not referring to translations. He’s talking about books written by frum yidden in English about Jewish topics (biographies of Rebbes, etc.). He also isn’t saying that they’re illegible because of bad spelling or grammar, he is saying that they are written with very clear, remedial literary skill.
And don’t just say “as long as everything is spelled right, what difference does it make?” I think, if ZD is right, it’s worth being upset about because it makes people think frum Jews don’t get anything beyond a sixth grade education in English.