HPF-84E For some reason, Thunderbird seems not to accept all language packs under /usr/lib64/thunderbird/extensions/
for some users.
In the thread mentioned in the beginning someone suggested to copy the language pack from /usr/lib64/thunderbird/extensions/
and add it manually to Thunderbird. And voilà! This works (although some things are not translated). I guess that it will work with the language pack you suggested.
But I still think that’s strange. The language packs are provided directly by Solus and are updated with every Thunderbird update.