Not my personal experience, but I've heard good things about Berlin and there are lots of young people living there, the air is very international. Barcelona is also a popular target of international students and people describe it as friendly. You have the Eastern Berlin and the Western Berlin, and the Eastern Berlin used to be the communist half, so it's less appealing, however, because of it, it's cheaper to rent a place to live there and lots of young people live there and it's more of a party district than Western Berlin. Spain is the south of Europe, so the atmosphere is more chill and people tend to socialise and have fun more. Vienna isn't a popular choice, so I haven't heard much of it.