This is by far the best way to translate selected text! Rated 5 out of 5 stars
I'm using this older add-on because I can't get it's successor to operate at all for me.
These are the only add-ons I've seen that preserve the markup (thus structure, format, links, etc.) while translating the selected text itself!
If you want to translate an entire page instead, you can use S3.Google Translator or gTranslator, which also preserve the markup while replacing the text inline within the page (not making you go to the google translate site).
Now if we could only have a single add-on that can do both in this way (selected text and full page).