How necessary would speaking fluent German be to live/work in Berlin or Vienna? I've heard that most young people and tech people speak English, but that it can be a problem, if you don't know good German, to interact with the civil service, banking, business registration, and those kinds of things. But I have no personal experience there.
One thing that makes it easy to stay in Scandinavia is that I've been rather spoiled by how easy it is to do everything in Denmark in English. Official government forms are available in your choice of Danish or English versions, all my banking is in English, doctors all speak it, and every government office I've been to will switch once they realize you're a foreigner. Heck, even many public lectures at universities and museums are in English. The language is good to know for social reasons, but there aren't really any formal/official situations where you have to interact in Danish.