Markov Dictionary Switcher 1.0 Requires Restart
by Freek Zindel
Based on Dictionary Switcher but more than a thousand times faster and you can choose dictionaries that you want to use. Detects the language of the message that you are composing and changes the spelling dictionary.
About this Add-on
After changing the dictionaries to use in the extension options, you might need to restart Thunderbird to exclude all the other dictionaries from switching.