Rated 2 out of 5 stars
It sometimes works, often it doesn't.
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.