The doctoral degree regulations for the specific faculty in which you wish to obtain a doctorate define whether the dissertation must be written in German or may be written in another language.
Please clarify with the dean of the faculty in which you wish to obtain a doctorate prior to applying for a place at the university whether the dissertation can be prepared in a language other than German.
If the dissertation is to be written in German, then you will need to demonstrate a high level of proficiency in German at the time of enrollment, specifically by submitting proof you have successfully completed the Deutsche Sprachprüfung für den Hochschulzugang ausländischer Studienbewerber (DSH) [German Website] or an equivalent.
If the dissertation is to be written in a language other than German, then the faculty in which you wish to obtain a doctorate must apply for an exemption for you from the Deutsche Sprachprüfung für den Hochschulzugang ausländischer Studienbewerber (DSH) at the International Preparatory and Language Center of JGU.
This exemption certificate must be presented at the time of your enrolment