Gmail lab announced a new feature called “Automatic Message Translation”. This integrates Google's automatic translation technology directly into Gmail. This lab feature will help you to translate messages you receive in other languages to your language. Simply enable "Message Translation" from the Labs tab under Settings, and when you receive an email in a language other than your own, Gmail will help you translate it into a language you can understand. In one click.
If all parties are using Gmail, you can have entire conversations in multiple languages with each participant reading the messages in whatever language is most comfortable for them.