It doesn't install for Thunderbird 11. I had to manipulate the install.rdf file.
I configured it to detect German, English and Spanish.
I had the spelling set to German and started typing in English. After a while the switcher switched to English. So I was happy. But then, after some more typing, it switched to Spanish. Hmm. Obviously one can set the correct language manually, so this sort of plugin is just a little icing on the cake, but sadly it ain't working.