I can only second what was already mentioned, it might have been a much better idea to just make use of Translate's auto-detection. This is what many, if not most people will (have to) resort to in the next step anyway.

At this point I guess it's really only useful (and probably thought) for those chiefly translating between a single language pair.

It would be convenient to write "auto" in the From filed !

I would be cool if we could set the from-language to 'auto', so it's a bit easier to use when encoutering multiple languages. Although setting a '*' seems to do the trick, most users won't find out about this.

Like Linux utilities, it's simple, directed to one function and works. I just won't use it, 'cause it's not the kind of addon I was searching for, I really enjoy more the small and quick popups for translation, not a new tab containing the function.
But it does work and it's as light as it should be. Very good improvement allowing the languages change.