Comment 56 for bug 545778

I'm looking at how this works upstream, and in Debian.

Upstream: the xpi that can be downloaded from https://addons.thunderbird.net/thunderbird/addon/lightning/ includes all locales. I don't know how this xpi is built though, given that the upstream build system insists on building a localized xpi that contains only one language at a time.

Debian: the extension is unpacked and installed in /usr/share/lightning, and each langpack package installs the corresponding translations in /usr/share/lightning/chrome/lightning/AB_CD/…
This works only because the extension is installed unpacked. Note that in Debian there's a separate source tarball for lightning translations (see https://packages.debian.org/source/buster/thunderbird), but maybe that predates the inclusion of lightning in the central repository?