Rated 5 out of 5 stars

The automatic recognition is far more handy than the dropdown list at the side of the Spelling button.
When you have time, if you can migrate it to Firefox Quantum ...

Not to forget : I learned in a forum that in recent versions of Thunderbird you have to go to the configuration editor, change "extensions.dictionary-switcher.autodetect" from false to true. It would be a good idea to write that in the documentation.