A brief glance at the websites of most research institutes and projects shows that English is the widely used lingua franca of German research. It is becoming increasingly common for research groups to work in international teams as part of international networks. Communicating in English is then usually taken for granted. At German institutes, especially in the natural and engineering sciences, researchers often research and also publish in English. Which language is used therefore depends on the subject area involved and the institute concerned. In many humanities and social science subjects, however, adequate knowledge of German is a requirement for a successful research stay. Depending on the research project, you may need to furnish proof of your language skills with your fellowship application.
Beyond that, however, there are good reasons for learning German: life also goes on outside the institute or campus. If you would like to get to know Germany and its inhabitants better, you should acquire basic knowledge of the German language – ideally before you arrive.