Unfortunately there are API changes in Firefox and Thunderbird 57 which break this add-on. I attempted to email a patch to the maintainer of the add-on, but my email bounced, so the add-on may be unsupported (and therefore permanently broken) at this point.
Switche la langue pour la page et le dico (instantanément), et l'interface complète (redémarre Fx). Gère des douzaines et douzaines de langues. Testé pour en_US, fr_FR et ar_EG sous Firefox 38.0.4 ESR.
Used to be a very good and almost essential add-on... However, it seems that the developers turned it into an adware / spyware during March, 2013. Developer's site (ppclick) seems to be a scam or ad-site as well – when it is working. The site has always been an advertiser one according to the archives. It is recommended to stay away from this add-on and the developer's website as well. Beware and do not install, but rather tweak and update some (clean) old version of the add-on for yourself.
Whether it works, depends on the version number and on the installed main language version. Sometimes it helps to install the "basic" English version and just add language packs manually, though even the information on the Mozilla site is contradictory regarding the install directory for XPI files (probably because this also changed over time).
Right now with thunderbird version 31.1.1 it does what it's expected under Linux but there seems no way to get it working under Win.
Still completely beyond comprehension why there are different language versions for one program in 2014(sic!). Despite the trouble I'd like to thank the developer for at least trying to provide a solution for something which was actually solved 20 years ago.