Language is the key to a country's culture. And even if it is possible to use English as a working language in many institutes and seminars at the university, for the purposes of leisure and every day life it is much easier if you speak at least a little German.
In Bonn, you can find several language schools that teach German as a foreign language.
For international researchers and their partners, the Welcome Center in cooperation with Pro-Integration offers German courses that cater to beginners and advanced learners. The courses are subject to a fee.
Furthermore, it is possible to learn a language in a tandem or online (e.g. at Deutsche Welle).