Hi Dave, almost simultaneously I got this message which provided the solution for me (after some technical hassle). So I’m happy, thank you for your support
At the moment, unfortunately, language packs for Thunderbird 78, are not yet available on addons.thunderbird.net and cannot be added from Tools > Options > General > Language & Appearance > Language > Set Alternatives…
If you want to add a language pack, you can download it manually from http://archive.mozilla.org/pub/thunderbird/releases/78.2.2/linux-x86_64/xpi/ and add it to the Add-ons Manager. It will show up under Languages. Once it is added, you can change to that language by going to Tools > Options > General > Language & Appearance > Language > Dutch > Toepassen en herstarten.
Even though you download it from linux-x86_64 that language pack will also work for Windows and Mac, because language packs are system independant.