Comment 18 for bug 1901829

I wonder if we can patch thunderbird to make it more robust in this kind of situation.

In the meantime, here is a workaround that should work for everyone affected:

    find ~/.thunderbird -name langpack*@thunderbird.mozilla.org.xpi -exec rm {} \;

Then if thunderbird isn't localized in your preferred language, install the corresponding Ubuntu package named thunderbird-locale-$LANGCODE.

Please let me know if this works for you.