I recommend you spend some time studying the language spoken at your destination before you go. I've learned some basic Italian, German, Russian, Norwegian, and French before various trips--and then sadly forgot much of it after the trip was over. (I tried Mandarin, but was pretty much a failure!) However, I have enjoyed the French enough, that I have continued to learn it for nearly two years now. I also recommend you pay careful attention to pronunciation in addition to vocabulary and grammar. In nearly every interaction I've had, the locals appreciate that I at least tried, and didn't show up totally unprepared speaking only English. Duolingo is a great resource, but you will need other books, idioms, and the like...
I also recommend you learn something of the customs, history, culture, food, etc before departing. Visiting a foreign country is not like going to Hawaii to veg out for a week; it is part of your education, and takes some work to prepare if you are going to get anything out of it (and not piss off the locals).
Some time spent on TripAdvisor forums will be very worthwhile, as will time reading Rick Steves (if you're going to Europe)! Eyewitness Travel guide are also great and very well illustrated.
And for dog's sake, leave PLENTY OF TIME at the airport when traveling internationally!!! On our most recent trip, It took us FOUR hours to get from curbside to our gate at CDG, but we were prepared because--as always--we leave plenty of time.
Also, don't try to do 21 cities in 20 days, or even 7 cities in 20 days. I advise you spend more time in fewer locations--take your time, unpack, relax, explore. No matter how much time and money you might have, you will never see it all. You don't want to spend your vacation packing and unpacking and figuring out new accommodations every day! We like to either rent a car and/or take train trips to various destinations near a major city. From Paris we took trips to Giverny, Chartres, Reims, Versailles, etc.
Also, PACK LIGHT. When you are lugging your stuff from car through airport to plane to taxi to train to bus to taxi, up narrow winding staircases, you will regret every extra ounce gram of crap you brought along!!!
As for safety, pickpockets are common in Europe, even sometimes in out-of-the-way locations, but in most EU countries violent crime is much less of a problem than in the U.S. Read Rick Steves' advice on this!