Technically, you don't need the know the language where you are going to make the program work. In reality, anything you do know about your hosts' language will most certainly help you a lot. Many of the families who request a Conversation Corps tutor have some background in English so most of our tutors can get by with basic communication. However, if you don't have any knowledge of the host county's language, taking a class before you leave or buying a program like Rosetta Stone is recommended. This will allow you to pick up the language faster when you are abroad and make you feel much more comfortable while on the program, especially when you are out and about on your own.
If you have ever studied a foreign language, you probably know that there is only so much you can learn from cramming text books and going through translation. In order to really become fluent, you have to constantly practice.
The ability to practice a second language is also a big motivator for Corps tutors. Many GeoVisions tutors have a lot of experience in a language and the Conversation Corps is a great, inexpensive way for them to really sharpen their skills. GeoVisions can also assist in finding in-country language classes while on the program. I'm here and always eager to help people through the process!
Here are some links you might find helpful:
http://www.esl-lounge.com (GeoVisions provides Premium Membership to this site when you are accepted into the Corps)
http://www.ehow.com/how_4806786_lesson-plans-teaching-english-abroad.html
http://www.julieinjapan.com/2007/10/english-lesson-plan-teaching-children.html