I served a full term as an English language teacher with the Peace Corps in Poland from 1997-1999 and had a fantastic time. You really don't need to worry about your language skills so much as your technical skills. For example, if you're planning to teach with the Peace Corps, you'd better have some teaching experience on your resume; if you hope to work with a local environmental non-profit, you should have some non-profit or environmental science experience. However, the Peace Corps doesn't expect their volunteers to be fluent in Swahili or Ukrainian when they sign up. In most countries, the Peace Corps runs training programs to teach their volunteers the local language and to familiarize them with daily life and potential difficulties in the host countries.
I arrived in Poland without any knowledge of Polish, but I had a decent grasp of it after 11 weeks of intensive lessons together with the other 84 members of my group. We also had lots of sessions about the work environment in Polish schools, a series of classes on Polish culture, and were advised about potential hazards. During the 11 week period, I lived with a host family and got to practice my Polish a lot. I was also initiated into Polish drinking rituals and ate more potatoes than I believed humanly possible.
I did apply to Peace Corps with a fair amount of teaching and tutoring experience. I think I may have been assigned to Poland because I speak German, but I can't be sure of that. I did not state any preference whatsoever for any particular country, but in general it is possible to specify a preference of a particular region or continent. The risk of doing so is that you may delay your application. Most countries only receive a new group of volunteers once a year. Your Spanish and potential Russian abilities will probably help you get to whatever part of the world you want to work in, but be advised that the types of programs are pretty limited in some parts of the world, and that there have been consistent problems with the Peace Corps program in Russia. In fact, just recently the Russians refused to renew a number of volunteers' visas, complaining that the volunteers lacked sufficient professional experience. Unfortunately, the programs in the Baltic States, Poland, Slovakia, and other parts of Central Europe have been closed--those countries are perceived as having graduated from the Peace Corps. Resources are now being allocated to countries in greater need.
I ended up teaching in a technical school in a town of 23,000 people in western Poland, and made a lot of friends with whom I still keep in contact. My Polish skills helped me get into a first-rate graduate program afterward, and I've found that a lot of people automatically assume that I'm a person of great virtue because I gave two years of my life to live in some hole earning a pittance. Actually, work was easy and I had all sorts of unofficial opportunities to travel. In my town I was a celebrity--most people were very kind to me. I really sacrificed very, very little to be there.
Hope that helps--I'll try to check this forum again later tonight or tomorrow in case you have other specific questions.
Yeah, there are somewhat similar British programs to the Peace Corps, some of which specialize in teaching (lang., business, or computer skills), but I forget their names. You can probably find some info at the local university or on the web somewhere.