For PhD and masters level, everything is in English except maybe some specific civil engineering courses about regulations. Also, I believe that for CS, everything is in English starting from Bachelors, with lectures in English and small problem classes being held either in German or English (you can choose).

Back when I was there, the first year of BSc was fully in German (apparently mandated by law), afterwards, there was the occasional lecture in German but usually with slides/exercises/exams available in English as well as English-speaking tutors.

There are also students from the French/Italian part of Switzerland, so there's a natural convergence towards English rather quick, at least that was my experience.

(take this with a grain of salt, it's been a while and my information might be outdated)