You can find Workarounds here: (https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1797945)
If you want the english Version of Lightning, type into a Terminal: sudo apt update sudo apt install xul-ext-lightning If you want a localized Version of Lightning, at the Moment, you cannot use the normal Add-On-Search in Thunderbird (incompatible). Download manually the latest Beta-Version of Lightining for your System- and Language-Version from: https://archive.mozilla.org/pub/calendar/lightning/candidates/ Open Thunderbird > Add-Ons > Extensions > (Remove unwanted english Version of Lightining) > > At the top of the page, click the Gear-Wheel-Button next to the Search-Box to open a menu > Install Add-on From File > Select Download-Folder and the .xpi-File > Install => All your Calendar-Data should be back -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to thunderbird in Ubuntu. https://bugs.launchpad.net/bugs/545778 Title: xul-ext-lightning is only available in English Status in One Hundred Papercuts: Triaged Status in Mozilla Thunderbird: Invalid Status in Ubuntu Translations: Triaged Status in lightning-sunbird package in Ubuntu: Invalid Status in thunderbird package in Ubuntu: Triaged Bug description: ******* ISSUE ******* HOW TO REPRODUCE 1. Change the user session language to other different than English. 2. In Thunderbird, open the calendar EXPECTED BEHAVIOUR - The calendar interface to be in the session language. REAL BEHAVIOUR - The calendar is in English. RELEVANT DETAILS - None. ********** SOLUTION ********** WORK-AROUND - None known. FIX - <http://www.smoose.org/blog/ubuntu-localized-lightning-extension>. REGRESSION POTENTIAL - Since no code needs to be modified, the regression potential is very low. **************** TECHNICAL INFO **************** Binary package hint: lightning-extension To manage notifications about this bug go to: https://bugs.launchpad.net/hundredpapercuts/+bug/545778/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp