Müller spricht Denglisch by Andra Riemhofer

Language barrier German English

Even if the Germans you are dealing with speak English at an advanced level, be aware that there is still plenty of room for linguistic misunderstandings. While a lot of people do have an excellent English vocabulary and great understanding of the language, they may feel uncomfortable if they are forced to engage in a live conversation. This often results from a lack of practice.
Take an “average” engineer like Herr Müller from Dr. Müller & Kollegen; he might be used to reading datasheets, specifications, or conference papers which are typically available only in English and may not have issues verbalizing facts in written form. But he could easily find it challenging when asked to converse in English in a face-to-face setting.
Consider hiring a local person like me to help you with doing research and setting up appointments; my experience is that Germans are more likely to engage in a brief conversation if they are called from a local number and the person calling knows the language and business culture.