When I was 19 I studied in Europe for six months. One thing that my dad did was find emergency references for me in the main countries I was visiting. These were work acquaintances, friend's family and others.
At first I went everywhere with friends. I planned a trip to Greece for the 3-week March break. I was to meet friends, but we were 19 and we messed up. So, I found myself in Greece on my own. At that point I decided to use the emergency reference, but they were out of town. But, it was fine. The concierge in the building took care of me. He called his daughter who spoke English, the found a room, they helped me search for my friends, and ultimately, I met up with the emergency contacts. But I toured on my own. I did send a postcard and when I got back to Germany, there was a message to call home, which I did. (The postcard could have been better worded! But I was only 19--your daughter has two years on me and at that age that is a lot! )
But then, I wanted to go to France and no one else did, so I planned my 10-day April mid spring term trip on my own. I stayed in hostels. I met other Americans and Canadians and Australians, and Irish etc. and made friends. I was fine. It was a fun time. I did not call home. I then took other trips on my own--Vienna and Rothenberg.
If she wants to go to clubs, she needs to stay in hostels and go with a group. And, she needs to watch the alcohol. If she will already have spent time in the UK and she has smarts, she will figure out how far she can push it. Would you be this worried if she were going to San Francisco clubbing? Is it the language concerns? So many of the people speak English. Prague is packed with young Brits. Of course, they are drinking. I don't think that she is relying on luck. If she is at all out-going she will meet people to clubbing with.
To help you sleep and to make this more like clubbing in SF, you might want to do is find some emergency contacts for her in Berlin and in Prague as well if you can. These can be friends of friends. You just need to tell them that you have confidence in your daughter, but that you are a parent and will sleep better if she has back up if needed. Then the question is your daughter's ability to judge what is an emergency. My emergency in Athens, could really have been solved by walking a few more blocks and using the American Express Card that my dad had also equipped me with. :) On the other hand you don't want her calling when she can't get a bus and has to pay for a taxi back to hostel. But if she is truly unsafe and a credit card and money won't get her out of it, then the emergency contact comes into play. That's when she would call you from San Francisco, right? :)