Where are you from? (doesn't really matter). Are you intent on WALKING everywhere, from Barcelona to Vienna to Amsterdam etc. Are you up to it, and doing it all with a heavy pack on your back (this is what kills these long distance hikes for me). Anyway, if you're fit yes it's feasible but I would be open to taking a train/bus sometimes, to save some time and give yourself a rest!
You don't need self defence lessons, though depending on where you plan to camp you might get into some interesting situations. Where did you expect to be camping? Some places have campsites but if you're planning on finding an out of the way place where you can get away with staying every night, this kind of thing can take a lot of your time. And in the major cities this is pretty much out. Sounds like you're not the type to worry too much about an itinerary, what's so helpful now (and even more so in a few years) is if you've got a smartphone, tablet or whatever you can plan and book things as you go.
Languages, most young people speak enough English to help you. Older people not so much but knowing a bit of French or German (depending where you are) is very common. I would strongly recommend not spending all your time in Western Europe, I go on holiday to Croatia, Bosnia-Hercegovina, Serbia, Romania and love these places. Southeastern Europe has a great climate and is not as expensive as most other places. Also Britain is nice (I have to say that).
Money is hard to say, obviously you can't have too much. If you're time rich and cash poor you can do some work on your way, seasonal farm work tends to done by non-locals, ie. grape picking, apple picking. A month of that, a caravan to stay in and no expenses, you could make a big difference to your funds that way. In Britain these workers get more than our minimum wage and get to do some real work, while the unemployed locals don't want to get their hands dirty. Off topic!
Anyway, a recent holiday in Croatia cost me and my girlfriend 50GBP each per day, including flights (from Britain) and accommodation (apartments costing 20-35GBP per night. Very little restaurant eating, mostly bread and cheese etc. from the market, travelling on buses.
Long story short, you can do it!