Hi, welcome to the forum

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What you read is correct and in order to know which language you must learn, first decide in which part of switzerland you want to live. I live in the german part and of course german is mandatory, I mean a lot of people speak english or at least understand it but if you learn german the doors open magically. I guess the same will be if you live in geneve and learn french or lugano and learn italian.
It might look that im not helping a lot, but i think in this you are the only one who can know whats the language, now if your question is which place will be better to live I live and love zurich.
In the other hand all over switzerland are lots of expat that share one thing, we all speak (or try to...) english. I need to make lots of calls to find services that manage english, so even with the language you already know could be great for expats looking for a nurse.