+1 I would love to have that button in the "Translate Selection into <language>" pop-up as it often offers to translate into the text's original language (e.g. Translate Japanese text from English into Japanese which doesn't make sense).
Google Translate has this button that I actually use quite often.
Additionally, it would be important if, when working with 3 or 4 languages, the list of recently used languages (let's say 5 languages, plus an `about:config` option for the number of languages) were displayed at the top as "Favorites" in both the source and target languages (this speeds up workflow).
I would also appreciate having "about:translations" as a button for a custom toolbar (opening in a new full-screen window).