Two more points that may help you.
1) You can prefix an individual command with a variable setting, to have that setting only affect that process.
2) In this case you don't need to set the overall LANG variable. It's the LC_TIME variable that controls the date format in thunderbird.
So try launching TB with the command like this:
Make sure that the desired locale is available on your system first, of course.
More on the time format in TB here: