Just like the this translation Greasemonkey script, Twanslate adds a simple button underneath any tweet that's in another language. You then click it anytime you want to convert it into English. The option to do this sits right below Twitter's star-shaped favorite button and works in your Twitter stream as well as an individual user's profile page., and
The two places where the translate option cannot be found are in Twitter's search result pages, as well as on individual tweets. This can be a bit of a downer, but is an easy fix if you're willing to click through to a user's profile.
The technology behind it is Google's translate service, which means you'll get a mix of very good, and sometimes comical, results. In any case, it's a nice site-specific add-on that you won't see or be bothered by until you need it.