There are no formal requirements for your English knowledge. If you have any certificates, please bring them with you. As your supervisor has to confirm that you are allowed to write your thesis, it is understood by this confirmation that he or she found your knowledge of English sufficient for carrying out your doctoral work.
If you feel like improving your English, there are a lot of courses at our University, e.g. at the FAU Graduate School (http://www.promotion.uni-erlangen.de/kurse-und-service/Interkulturalitaet/) or at FAU Language Centre.