having lived in Switzerland for 5 years, part of that time working for the Oliver Wyman Zurich office, normally the requirements are:
(1) Graduate hires: C1 German skills as a lot of clients require teams to speak / work in German, create slides in German and so forth (as a matter of fact, many Swiss clients even prefer Swiss German speaking consultants over German speaking consultants (and yes, Swiss German is an entirely different language). The only exception I encountered was McK as they have an office in Geneva, do a lot of business there and accept French-speaking candidates. However, if you are not fluent in either of the two languages, you should consider investing some time in your German skills - they will test this during your interviews, so it doesn't help simply to state it in your CV.
(2) Experienced hires: No German skills required if you are hired for your expert skills or network and they can afford sending you only to english speaking clients. I have met Italian- / English-speaking, French- / English-speaking and English only-speaking principals / partners working for Swiss offices.
Hope this helps!