Gmail: Translate on the Fly

There is an option under settings to have all emails translated into your default language, but why stop there? Why not have an optional drop down language selector for any recieved, or composed, email. So I should be able to type an email in English, and translate it into Spanish when I send it. And translate the Spanish reply, into English. This would eliminate the need for another translator, such as Babel Fish, or Google Translate.