Review for Dictionary Switcher by peci1
Rated 3 out of 5 stars
As the previous review says, only one line needs to be changed to make this addon work in recent TB versions :)
But what he suggests is for making it work in Firefox. To make it work in Thunderbird, change the same line, but in file mail.js (inside dictionary-switcher.jar!/chrome/content).
To create your own collections, you must have a Mozilla Add-ons account.