kyrios
That's exactly what's so frustrating - I don't know why!
activeindolence
When I follow your path and try to select German as an alternative language it simply isn't in the list of installed languages, so I cannot pick it.
When I go to Preferences > Extensions & Themes and try to 'manage my languages' it isn't there.
So, I type "German language pack" in the search box and find only incompatible versions (like the one I used to have, that got deactivated) or dictionaries.
Installing one of those dictionaries doesn't help.
I presume that something got deleted during the update process of Thunderbird (last Friday, July 17).
Yet, shouldn't I be able to install a language pack manually?
When I search the Thunderbird or Mozilla websites for it I only find incompatible packs or ancient search results, support for Windows XP and such. Not exactly helpful... 😀