I downloaded the dictionary (not the language pack!) as an .xpi file from here:
https://addons.thunderbird.net/en-US/thunderbird/addon/german-dictionary/ and installed it by using
Install Addon from File in the small gear menu in the addons manager. The dictionary is showing up in my installed Extensions.
The problem is that I still cannot choose German as a language for spell checking, neither when composing a new mail, nor under
Tools -> Options -> Composition -> Spelling. I tried it with several dictionaries, same result. How can I solve this?
I’m on Windows 10 and Thunderbird 52.9.1 (32-bit).