Comment 12 for bug 1637801

I talked with Martin Pitt, who let me know that it's intentional behavior that gettext gives precedence to /usr/share/locale over /usr/share/locale-langpack.

apt is one of those 'special' packages in main where the translations shipped with the source package are not stripped at build time. It means that the apt.mo files installed by the language packs are not effective (as we have already figured out). So in case of apt (and a few other special packages) translation updates need to go into the package to make a difference to the users.

@Julian: The Xenial template in Launchpad seems to have been updated 2016-03-25 (attached). The easiest (I think) way to get the actual translations is to download them from Launchpad, and simply replace the PO files in the package.

https://translations.launchpad.net/ubuntu/xenial/+source/apt/+pots/apt

There shouldn't be a need to manually merge as you described. (Possibly I have missed something.)