Also has a how-to for doing it manually.

” … Switching dictionaries manually every time you write in a different language can be quite the nuisance if you have to do so several times a day.
“The Thunderbird extension Automatic Dictionary Switching makes life a lot easier by remembering the selected dictionary for each contact that you have in the browser. It saves the selection that you make under spelling when you are in a compose window so that it will be remembered and automatically selected the next time you write that particular user or users a messages. … “

Source: Ghacks Technology Newsletter
Details  Switch dictionaries automatically based on recipients