Thank you, this was exactly my problem, too. So IMHO the thunderbird-l10n-$(lang) packages should recommend the corresponding lightning-l10n-$(lang) package, since thunderbird recommends lightning.
(I used to only "apt install thunderbird-l10n-de", which pulled in thunderbird and lightning by depends/recommends, and just now wondered where my calendar is ;-) ) Even better would be to detect the missing lightning-l10n-pack and either let lightning fallback to english or, if that is somehow not possible, notify the user. In the current state, there is no indication what is wrong: - lightning is shown in the addon-manager - no error is shown - but all artifacts of lightning are completely missing: - no icons in the toolbar - no menu-entry for the whole calendar-menu - no "calendar"-tab in settings - no calendar-related items in File -> new (no "new calendar", "new event", "new task" etc.)
