Comment # 30 on bug 939153 from Frank Kruger

(In reply to Timo Sigurdsson from comment #29)
> I'd like to chime in here. This is still an issue with Leap 15 - and more
> importantly, with the release of Thunderbird 60, it's not as easy to work
> around anymore, since Mozilla doesn't seam to publish the Lightning xpi
> estension file anymore (or at least not in an easy to find location).
> 
> So, I had to download a release tarball of Thunderbird 60 and extract the
> localized extension package from there. This is certainly anything but
> user-friendly.
> 
> What strikes me, though, is the fact that I can see the official Thunderbird
> packages do seem to contain localization files for the Lightning Calender in:
> /usr/lib64/thunderbird/extensions/langpack-de@thunderbird.mozilla.org
> (my locale is 'de')
> 
> But how do I get Lightning to actually make use of these translations?

For the time being, as a workaround, you may use
https://ftp.mozilla.org/pub/calendar/lightning/candidates/6.2b6-candidates/build1/linux-x86_64/