Do you write emails in multiple languages? You'll be pleased to know we've finished an enhancement that enables MULTIPLE Dictionaries to be active in Thunderbird, simultaneously!
This feature should land in the Beta for 102 in a few weeks, and ultimately in the Stable 102 release at the end of June.
Here's the Bugzilla link: https://bugzilla.mozilla.org/show_bug.cgi?id=1761221