Updated.
As the same-origin policy restriction on the <canvas> object hamstrings the eyedropper function rather convincingly, I've removed it entirely; if anyone has any workaround for this which can be executed in the context of a user script (or if it was still working for you somehow), let me know and I will gladly re-add it.
The major feature addition is the 'Special Characters' button, which facilitates insertion of some common special characters (♡♥☆★♪♫♂♀「」) into note text. Please let me know if there are any other useful characters which should be added; if I'm too slow about it, the list of special characters has been pulled to the top of the script so you can add them, or indeed any string of your choice, to the menu easily. (The background color for translation mode has also been pulled to the top for your convenience, if you find the new mild, pleasant turquoise disconcerting and would like to return to the old searing, garish green.)
The 'Save as Translated' button now sends another AJAX request to read tags from the image at the time the button is pressed, rather than reading from the HTML of the cached page. This means that using the script on posts like the Kogasa comics, which often have lots of tags added to them between the time the page is loaded and the time the translation is finished, will no longer result in a whole bunch of tags being inadvertently deleted. The regexes have also been fixed to work properly for some edge cases.
Thanks to RaisingK, Ars, and Moonspeaker for elucidating the nature of these and other bugs.