Hi Khaled,
thank you so much for working on this! Please see bug
https://bugzilla.mozilla.org/show_bug.cgi?id=1117714 for an example on
how the process goes. Some of those steps don't apply to you since you
are already translating Thunderbird.
What might be a bit confusing about Lightning is that there is a
dashboard at
https://l10n.mozilla.org/shipping/dashboard?tree=cal_beta
but you should NOT sign-off on this one.
Instead, just sign-off on the Thunderbird dashboard as usual, and
Lightning will be included.
Let me know if you have questions
Philipp