As an employee of Credit Suisse I suppose I can answer this question.
English is the working language globallz of Credit Suisee, and you will find that many of the communications from the bank are in English. (The AGM is conmmunicated by OJ et al in German) I would comment that the difference in the expectation of languages comes from the culture of any international company.
Credit Suisse is proud to be a Swiss company, and the very senior managers of the bank are generally Swiss, and therefore many meetings are conducted in Swiss German and even High German is seen to be a departure from the comfort level of many (especially in private banking).
You may find some comfort that the Head of Investment Banking is American and this is largely due to the fact that the major IB operation is in the US.
To work for CS in Zürich other languages are seen as an important pre-requisite, apart from a lucky few, myself included.
I have Swiss friends that work for other IB institutions in Zürich, and would recommend that you try some of the other (non-Swiss) investment companies too, such as Nomura, or any of the American companies that have a presence here, a few of which I know are actively hiring.
Credit Suisse is a great copmpany to work for and I think a job here, or in any country is a great opportunity.
Generally, if you are a foreigner in Zürich and would like a job, and/or an apartment here that is in demand, you would be expected to make a real effort to explain your reasons for why you deserve it over the Swiss, and any other German speaking foreigners.
I think a phone call or any personal CV explaining your background and how you can empathise with the position of the employee/landlord in why they would prefer another candidate, but why you are more deserving and can be a better fit.
It worked for me, so can for anyone.
Hope this helps.
PS a bit of Vitamin B can help too