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(take this with a grain of salt, it's been a while and my information might be outdated)