Advertising on the internet even in America and you might get people who only speak Spanish. You need to know what they say when they reply and you need to be able to translate what you are saying if you want to continue the conversation. While automatic translation isn't perfected yet, there are some free services that make it pretty easy to get close.
I recommend http://www.google.com/language_tools?hl=en
I just recently had this situation and being able to translate back and forth between English and Spanish has been helpful.