I taught for a while in Spain and let me tell you that although I had 8 years of Spanish in school and got high marks, when I got to Spain I couldn't speak or understand a word.
I was offered my first ESL job at a school in Seville and was worried at first because I couldn't speak the language. Worse was that the folks in southern Spain have an accent where they not only speak fast but don't pronouce all the letters especially the endings of words. Eesh!
At first their Spanish didn't sound like Spanish at all.
Anyway, the dean told me that it was good that I didn't speak the language because the students would have to rely on what they knew of English to communicate with me. So not speaking Spanish was an advantage.
I had a class of adults and one of 12 year olds. The 12 year olds was another story altogether, an enlightening one of going from failure and misery to one of un gran exito! -- a resounding success. (To read that story go to:
http://www.english-teaching-info.com/te ... ldren.html
As time went on I did pick up a lot of Spanish and became quite fluent. Today when I teach a Spanish speaker I use the language to help translate but I don't know if this is necessarily a good idea. It makes it easier for me but I'm not sure if that is the best for the student.
Hope this helps.