It is possible to become a historian without being able to read the original sources, however, this heavily varies on what your specialization is. If your focus is Nazi Germany during World War II, you could manage without knowing German, French, Polish, etc as this field is many documents from this era have been translated. However, if your field is more specific and less studied you absolutely need to learn the language of the original sources as you will have to translate many documents. Even If you plan on becoming a historian with a specialization in a well known era, you should still learn the original language as it will make you more appealing when you apply for jobs. The job market for historians is small and extremely competitive, you must get every advantage you can over another person for applying for the same positions as you. So while another language may not be a requirement it is heavily recommended.