Anne (no prob re this post, I'll move it to the talk thread later on, when you've received all replies), can't add much to Mark's great post.
I'd go for Spain (pref. Barcelona) if you're interested to learn a most useful language, combined with the opportunity of a lot of culture and nice scenery (and good weather
I would pick France as second option.
If you'd like things on a smaller scale, and would want to learn French though, I'd go for Belgium then (Brussels, or the Walloon cities of Liège, Namur), as Axel is proposing. It's a slightly different French sometimes LOL, BUT Brussels is so cosmopolitan and at the same time pretty small. Liège has the big advantage of being located in an extremely nice looking area (if you like hiking, skiing, water sports), and is so close to the German border and Dutch Limbourg, which is a huge plus to get an idea of these regions too. Namur is small and very nice too, also a great scenery there and lots of outdoor opportunities as well.
IF you don't mind about learning useful languages too much, I'd go for Flanders then. First of all, it's quite practical as you'll find more people able to speak English than in the Walloon part of Belgium.
The best cities would be Leuven, Antwerp or Gent then. Leuven is a fantastic place, THE university city of Flanders, full of students everywhere (and looooots of pubs
). Antwerp and Gent are also very attractive for a student, always something going on there as well. All three cities have a huge historic background, it's full of culture and you won't be bored for a minute.
I'll stop my Belgium-promo here, I suppose
Good luck with your search!