What are good reasons to write my thesis in English?
Clearly, in the time you have performing research and interacting with other people in your field, you were using at least one language, likely more than one given your research on a country that (likely) does not speak either English or German. However, your thesis has some specific audiences to address. If written in German, it certainly addresses your local university department, and the academic community in Germany. Writing it in English will dramatically broaden the potential audience, since many more people in the world can read English as compared with German. But, do those additional people count much? That is up to you to determine, and the university to agree to.

Look at your references, fellow academics you've met at conferences, and those you wish to interact with. Write in the language most likely to be accessible to them.