Complex sentence: He supposed that the main reason for the hate and prejudice lay in mutual misunderstanding, caused by the lack of one common language that would play the role of a neutral communication tool between people of different ethnic and linguistic backgrounds.
Split into simpler sentences: He supposed that the main reason for the hate and prejudice lay in the mutual misunderstanding caused by the lack of one common language. If such a language existed, Zamenhof postulated, it could play the role of a neutral communication tool between people of different ethnic and linguistic backgrounds.